<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with Wdesk from Workiva-->
<!--Copyright 2022 Workiva-->
<!--r:5821af8e-2b97-40a1-a6b6-c5356556d8aa,g:67357f82-1bed-4e02-ba37-f8094dd8f8a6-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://thredup.com/role/CoverPage" xlink:type="simple" xlink:href="tdup-20211231.xsd#CoverPage"/>
  <link:definitionLink xlink:role="http://thredup.com/role/CoverPage" xlink:type="extended" id="i1a239779ca2544869f3bc660c7858979_CoverPage">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_7ddb2bef-3c4e-46bb-9ab2-15f72fc6c97c" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_DocumentType_7ddb2bef-3c4e-46bb-9ab2-15f72fc6c97c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentAnnualReport_b2553fa5-6447-461c-a32d-0b626bcd4c25" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentAnnualReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_DocumentAnnualReport_b2553fa5-6447-461c-a32d-0b626bcd4c25" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_1ad95763-f7ba-4a61-b4cd-b71ed94ef75a" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_DocumentPeriodEndDate_1ad95763-f7ba-4a61-b4cd-b71ed94ef75a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_cb5ce4a8-bf8d-4e98-8a7f-5643f5ce1251" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_CurrentFiscalYearEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_CurrentFiscalYearEndDate_cb5ce4a8-bf8d-4e98-8a7f-5643f5ce1251" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_b69bb151-504f-4b2e-96dd-67534db315eb" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentTransitionReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_DocumentTransitionReport_b69bb151-504f-4b2e-96dd-67534db315eb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_a552b83d-dea5-4152-8c6f-1db3a7fa5e9c" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityFileNumber_a552b83d-dea5-4152-8c6f-1db3a7fa5e9c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_0781d393-a966-46d7-92a0-5ec9652f2a66" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityRegistrantName_0781d393-a966-46d7-92a0-5ec9652f2a66" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_edef3fc2-dfdd-4003-b1c1-29ff8e06e655" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityIncorporationStateCountryCode_edef3fc2-dfdd-4003-b1c1-29ff8e06e655" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_6f781842-8e03-4d03-bbf0-4126f85ee657" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityTaxIdentificationNumber_6f781842-8e03-4d03-bbf0-4126f85ee657" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_295c332a-5836-4ab7-889d-1c3cc27b6f77" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityAddressAddressLine1_295c332a-5836-4ab7-889d-1c3cc27b6f77" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine2_dd9b0d6e-e999-4fef-870f-4c76b5b0cbec" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityAddressAddressLine2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityAddressAddressLine2_dd9b0d6e-e999-4fef-870f-4c76b5b0cbec" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_a9dad860-7fca-4244-b735-4ca74ba951f8" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityAddressCityOrTown_a9dad860-7fca-4244-b735-4ca74ba951f8" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_b221ee60-0de8-400b-b5cd-47028f7fc20b" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityAddressStateOrProvince_b221ee60-0de8-400b-b5cd-47028f7fc20b" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_f9f40dd1-4a54-4890-a212-d54866330df6" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityAddressPostalZipCode_f9f40dd1-4a54-4890-a212-d54866330df6" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_e9e248ad-261d-49fc-bfea-0c403505b9d2" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_CityAreaCode_e9e248ad-261d-49fc-bfea-0c403505b9d2" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_ad041619-2fc3-4422-9c82-11805038ad2d" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_LocalPhoneNumber_ad041619-2fc3-4422-9c82-11805038ad2d" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_fd678e20-a2cb-4325-ac1e-4feee96ed695" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_Security12bTitle_fd678e20-a2cb-4325-ac1e-4feee96ed695" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_1247a51f-4118-455c-a2f0-4221134fd30e" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_TradingSymbol_1247a51f-4118-455c-a2f0-4221134fd30e" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_92866bda-7afc-458a-9586-f1c15bbace73" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_SecurityExchangeName_92866bda-7afc-458a-9586-f1c15bbace73" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityWellKnownSeasonedIssuer_4484bfd9-89c4-4ba5-87f7-0c63555e4667" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityWellKnownSeasonedIssuer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityWellKnownSeasonedIssuer_4484bfd9-89c4-4ba5-87f7-0c63555e4667" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityVoluntaryFilers_ba7edb5d-8df1-4f91-aad6-fceb554e65b2" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityVoluntaryFilers"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityVoluntaryFilers_ba7edb5d-8df1-4f91-aad6-fceb554e65b2" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_ef234759-8794-4a1b-b049-25fe9982251c" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityCurrentReportingStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityCurrentReportingStatus_ef234759-8794-4a1b-b049-25fe9982251c" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_ec467fc4-68f1-498c-910f-c43e10cf31da" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityInteractiveDataCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityInteractiveDataCurrent_ec467fc4-68f1-498c-910f-c43e10cf31da" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_7ac0153b-d9ae-4953-9284-639fa4c5b6ba" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityFilerCategory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityFilerCategory_7ac0153b-d9ae-4953-9284-639fa4c5b6ba" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_5e84500a-f444-4f70-9ec6-c5b11c4398a4" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntitySmallBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntitySmallBusiness_5e84500a-f444-4f70-9ec6-c5b11c4398a4" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_d480608f-8883-447d-8050-f8eca2c0149f" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityEmergingGrowthCompany_d480608f-8883-447d-8050-f8eca2c0149f" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityExTransitionPeriod_c180ad69-bd7f-46b2-87f3-4079ba9cb323" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityExTransitionPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityExTransitionPeriod_c180ad69-bd7f-46b2-87f3-4079ba9cb323" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_IcfrAuditorAttestationFlag_49d4272a-1208-40be-bdc5-a0809f5a02d4" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_IcfrAuditorAttestationFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_IcfrAuditorAttestationFlag_49d4272a-1208-40be-bdc5-a0809f5a02d4" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_04fbb3ad-beeb-4763-8b97-e4dd3ae33663" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityShellCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityShellCompany_04fbb3ad-beeb-4763-8b97-e4dd3ae33663" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityPublicFloat_242def51-d78c-4ed0-9d97-53aef8478374" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityPublicFloat"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityPublicFloat_242def51-d78c-4ed0-9d97-53aef8478374" xlink:type="arc" order="29"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_f0d1ceda-a1c1-4ec0-950a-b2a3751abdc4" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityCommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_f0d1ceda-a1c1-4ec0-950a-b2a3751abdc4" xlink:type="arc" order="30"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentsIncorporatedByReferenceTextBlock_ad0693ab-cadf-4d53-80dd-b6c7b2782d68" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentsIncorporatedByReferenceTextBlock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_DocumentsIncorporatedByReferenceTextBlock_ad0693ab-cadf-4d53-80dd-b6c7b2782d68" xlink:type="arc" order="31"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_2ede5151-4e25-42f2-af2c-25f8ef3aa6af" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_AmendmentFlag_2ede5151-4e25-42f2-af2c-25f8ef3aa6af" xlink:type="arc" order="32"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_d5ba8bac-42b5-4e6a-9cd7-df03cf6f48ac" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentFiscalYearFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_DocumentFiscalYearFocus_d5ba8bac-42b5-4e6a-9cd7-df03cf6f48ac" xlink:type="arc" order="33"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_15d57b49-b1e0-4ee7-9f70-75565e5f96fc" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentFiscalPeriodFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_DocumentFiscalPeriodFocus_15d57b49-b1e0-4ee7-9f70-75565e5f96fc" xlink:type="arc" order="34"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_924ba2d9-1450-487b-95ba-2ade14a0ba95" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_EntityCentralIndexKey_924ba2d9-1450-487b-95ba-2ade14a0ba95" xlink:type="arc" order="35"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_db4d40f8-04a3-41a4-978f-bf3a8e74e36a" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_DocumentInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_907cfc5b-df73-4457-a9d0-0403e74660af" xlink:to="loc_dei_DocumentInformationTable_db4d40f8-04a3-41a4-978f-bf3a8e74e36a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_b799c67b-eeec-4294-8a55-e6f45d5f7afa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_db4d40f8-04a3-41a4-978f-bf3a8e74e36a" xlink:to="loc_us-gaap_StatementClassOfStockAxis_b799c67b-eeec-4294-8a55-e6f45d5f7afa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_b799c67b-eeec-4294-8a55-e6f45d5f7afa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_b799c67b-eeec-4294-8a55-e6f45d5f7afa" xlink:to="loc_us-gaap_ClassOfStockDomain_b799c67b-eeec-4294-8a55-e6f45d5f7afa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_cadd08fa-c907-412a-b85e-e071614678fc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_b799c67b-eeec-4294-8a55-e6f45d5f7afa" xlink:to="loc_us-gaap_ClassOfStockDomain_cadd08fa-c907-412a-b85e-e071614678fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_d9d21b5d-7eef-4176-964a-4d9e0e856e48" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_cadd08fa-c907-412a-b85e-e071614678fc" xlink:to="loc_us-gaap_CommonClassAMember_d9d21b5d-7eef-4176-964a-4d9e0e856e48" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_b8df1884-5f83-44f4-8845-ee872b0862d4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_cadd08fa-c907-412a-b85e-e071614678fc" xlink:to="loc_us-gaap_CommonClassBMember_b8df1884-5f83-44f4-8845-ee872b0862d4" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/ConsolidatedStatementsofOperations" xlink:type="simple" xlink:href="tdup-20211231.xsd#ConsolidatedStatementsofOperations"/>
  <link:definitionLink xlink:role="http://thredup.com/role/ConsolidatedStatementsofOperations" xlink:type="extended" id="i8d865b03d493426baef7c06e5b55e729_ConsolidatedStatementsofOperations">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_586de64e-017f-4034-8e90-e4e473a5ed69" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesAbstract_a111eed6-517d-4043-8de2-5d43645a36ba" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenuesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_586de64e-017f-4034-8e90-e4e473a5ed69" xlink:to="loc_us-gaap_RevenuesAbstract_a111eed6-517d-4043-8de2-5d43645a36ba" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_0274e06d-6edd-44c1-8ba7-a097272d31e3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesAbstract_a111eed6-517d-4043-8de2-5d43645a36ba" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_0274e06d-6edd-44c1-8ba7-a097272d31e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfRevenueAbstract_dbb617bc-9a51-407e-a895-f9f4f6909c19" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CostOfRevenueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_586de64e-017f-4034-8e90-e4e473a5ed69" xlink:to="loc_us-gaap_CostOfRevenueAbstract_dbb617bc-9a51-407e-a895-f9f4f6909c19" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfRevenue_2b370fcd-2206-4e47-bf1b-755d8eaf93fa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CostOfRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostOfRevenueAbstract_dbb617bc-9a51-407e-a895-f9f4f6909c19" xlink:to="loc_us-gaap_CostOfRevenue_2b370fcd-2206-4e47-bf1b-755d8eaf93fa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GrossProfit_eeb0c865-9ed1-44ca-baa5-573234c002f3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GrossProfit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_586de64e-017f-4034-8e90-e4e473a5ed69" xlink:to="loc_us-gaap_GrossProfit_eeb0c865-9ed1-44ca-baa5-573234c002f3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostsAndExpensesAbstract_5a9b4289-a2a0-44f1-9e3d-24c0187198da" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CostsAndExpensesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_586de64e-017f-4034-8e90-e4e473a5ed69" xlink:to="loc_us-gaap_CostsAndExpensesAbstract_5a9b4289-a2a0-44f1-9e3d-24c0187198da" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_OperationsProductAndTechnologyExpense_8846e1ff-32e8-4647-b077-6d6673a9abff" xlink:href="tdup-20211231.xsd#tdup_OperationsProductAndTechnologyExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_5a9b4289-a2a0-44f1-9e3d-24c0187198da" xlink:to="loc_tdup_OperationsProductAndTechnologyExpense_8846e1ff-32e8-4647-b077-6d6673a9abff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketingExpense_4fbe9a32-4477-4dda-9687-371f21da8c90" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_MarketingExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_5a9b4289-a2a0-44f1-9e3d-24c0187198da" xlink:to="loc_us-gaap_MarketingExpense_4fbe9a32-4477-4dda-9687-371f21da8c90" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpense_454fa43c-931e-4d53-a34d-762f2a5790bd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_5a9b4289-a2a0-44f1-9e3d-24c0187198da" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpense_454fa43c-931e-4d53-a34d-762f2a5790bd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpenses_f45a12a7-b361-49f8-9edb-5ed29734899e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_5a9b4289-a2a0-44f1-9e3d-24c0187198da" xlink:to="loc_us-gaap_OperatingExpenses_f45a12a7-b361-49f8-9edb-5ed29734899e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_0bd37ddf-a302-4016-b13e-aa3f6ec31e52" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_586de64e-017f-4034-8e90-e4e473a5ed69" xlink:to="loc_us-gaap_OperatingIncomeLoss_0bd37ddf-a302-4016-b13e-aa3f6ec31e52" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_f5308858-cf2a-43a1-9c65-c38fcda12e95" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_586de64e-017f-4034-8e90-e4e473a5ed69" xlink:to="loc_us-gaap_InterestExpense_f5308858-cf2a-43a1-9c65-c38fcda12e95" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_eabaaba6-7c1c-4e17-b19a-10f86b521f56" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_586de64e-017f-4034-8e90-e4e473a5ed69" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_eabaaba6-7c1c-4e17-b19a-10f86b521f56" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_73351d47-647e-495b-a5e4-df828a22570f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_586de64e-017f-4034-8e90-e4e473a5ed69" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_73351d47-647e-495b-a5e4-df828a22570f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_fd2d772e-cb06-4567-8926-e7446a4468d2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_586de64e-017f-4034-8e90-e4e473a5ed69" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_fd2d772e-cb06-4567-8926-e7446a4468d2" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_21850450-5344-4e75-8834-5267bce944bd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_586de64e-017f-4034-8e90-e4e473a5ed69" xlink:to="loc_us-gaap_NetIncomeLoss_21850450-5344-4e75-8834-5267bce944bd" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_3da1eea7-79d7-418f-b91e-2e6509b0f6e7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_586de64e-017f-4034-8e90-e4e473a5ed69" xlink:to="loc_us-gaap_EarningsPerShareDiluted_3da1eea7-79d7-418f-b91e-2e6509b0f6e7" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_974d5964-54a1-4538-ac2e-62e4147b912c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_586de64e-017f-4034-8e90-e4e473a5ed69" xlink:to="loc_us-gaap_EarningsPerShareBasic_974d5964-54a1-4538-ac2e-62e4147b912c" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_a41e5668-8956-4d00-9298-9b0154e62a81" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_586de64e-017f-4034-8e90-e4e473a5ed69" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_a41e5668-8956-4d00-9298-9b0154e62a81" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_105d3a70-484a-462a-bbff-54cdc1c0997a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_586de64e-017f-4034-8e90-e4e473a5ed69" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_105d3a70-484a-462a-bbff-54cdc1c0997a" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_167480fa-77c6-4883-9cd2-f7adfb42eb32" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_586de64e-017f-4034-8e90-e4e473a5ed69" xlink:to="loc_us-gaap_StatementTable_167480fa-77c6-4883-9cd2-f7adfb42eb32" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_dd655331-4885-4080-8700-6a79ce2173d9" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_167480fa-77c6-4883-9cd2-f7adfb42eb32" xlink:to="loc_srt_ProductOrServiceAxis_dd655331-4885-4080-8700-6a79ce2173d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_dd655331-4885-4080-8700-6a79ce2173d9_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_dd655331-4885-4080-8700-6a79ce2173d9" xlink:to="loc_srt_ProductsAndServicesDomain_dd655331-4885-4080-8700-6a79ce2173d9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_64ead25e-6038-49b1-8313-5bd261b9830c" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_dd655331-4885-4080-8700-6a79ce2173d9" xlink:to="loc_srt_ProductsAndServicesDomain_64ead25e-6038-49b1-8313-5bd261b9830c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ConsignmentMember_b79b31d4-a690-431e-a0c5-4e276069e6bd" xlink:href="tdup-20211231.xsd#tdup_ConsignmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_64ead25e-6038-49b1-8313-5bd261b9830c" xlink:to="loc_tdup_ConsignmentMember_b79b31d4-a690-431e-a0c5-4e276069e6bd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductMember_0f58009e-e22b-4c30-870a-89be2c152765" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProductMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_64ead25e-6038-49b1-8313-5bd261b9830c" xlink:to="loc_us-gaap_ProductMember_0f58009e-e22b-4c30-870a-89be2c152765" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/ConsolidatedStatementsofConvertiblePreferredStockandStockholdersEquity" xlink:type="simple" xlink:href="tdup-20211231.xsd#ConsolidatedStatementsofConvertiblePreferredStockandStockholdersEquity"/>
  <link:definitionLink xlink:role="http://thredup.com/role/ConsolidatedStatementsofConvertiblePreferredStockandStockholdersEquity" xlink:type="extended" id="i1f691ababaf443898f5bd0df32588b32_ConsolidatedStatementsofConvertiblePreferredStockandStockholdersEquity">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_f79a8ecb-00bd-448e-b40c-fe6851662c35" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_a378968c-31ce-4843-89a6-1edb8504d968" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncreaseDecreaseInTemporaryEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f79a8ecb-00bd-448e-b40c-fe6851662c35" xlink:to="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_a378968c-31ce-4843-89a6-1edb8504d968" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesOutstanding_4b14790d-4a55-4f67-8ab1-a7ecc8ddebe0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TemporaryEquitySharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_a378968c-31ce-4843-89a6-1edb8504d968" xlink:to="loc_us-gaap_TemporaryEquitySharesOutstanding_4b14790d-4a55-4f67-8ab1-a7ecc8ddebe0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_2a691e90-f582-40c8-9dd9-2d100397dedb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TemporaryEquityCarryingAmountAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_a378968c-31ce-4843-89a6-1edb8504d968" xlink:to="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_2a691e90-f582-40c8-9dd9-2d100397dedb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_TemporaryEquityConversionOfConvertibleSecuritiesShares_1b47a626-be76-4756-a166-4a028eb3a76c" xlink:href="tdup-20211231.xsd#tdup_TemporaryEquityConversionOfConvertibleSecuritiesShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_a378968c-31ce-4843-89a6-1edb8504d968" xlink:to="loc_tdup_TemporaryEquityConversionOfConvertibleSecuritiesShares_1b47a626-be76-4756-a166-4a028eb3a76c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_TemporaryEquityConversionOfConvertibleSecuritiesValue_cf2b49d3-40f1-41e8-aff8-77e1f46716b3" xlink:href="tdup-20211231.xsd#tdup_TemporaryEquityConversionOfConvertibleSecuritiesValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_a378968c-31ce-4843-89a6-1edb8504d968" xlink:to="loc_tdup_TemporaryEquityConversionOfConvertibleSecuritiesValue_cf2b49d3-40f1-41e8-aff8-77e1f46716b3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_TemporaryEquityStockIssuedDuringPeriodSharesNewIssues_58d3ca48-9678-471b-a912-932e848b6aac" xlink:href="tdup-20211231.xsd#tdup_TemporaryEquityStockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_a378968c-31ce-4843-89a6-1edb8504d968" xlink:to="loc_tdup_TemporaryEquityStockIssuedDuringPeriodSharesNewIssues_58d3ca48-9678-471b-a912-932e848b6aac" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityStockIssuedDuringPeriodValueNewIssues_db864790-289c-4d27-a3ef-fc8dc9f9571e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TemporaryEquityStockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_a378968c-31ce-4843-89a6-1edb8504d968" xlink:to="loc_us-gaap_TemporaryEquityStockIssuedDuringPeriodValueNewIssues_db864790-289c-4d27-a3ef-fc8dc9f9571e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesOutstanding_76c0f17d-cb3c-4d18-8456-4880d847c370" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TemporaryEquitySharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_0dcf3f20-d78c-49ff-b741-aef281ef1ff2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TemporaryEquityCarryingAmountAttributableToParent"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f79a8ecb-00bd-448e-b40c-fe6851662c35" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_3a997add-7693-4cc0-b76d-8217a4b8e46f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_3a997add-7693-4cc0-b76d-8217a4b8e46f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_bb734f8e-c2fb-40f6-985c-ebea0e2716c8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_us-gaap_StockholdersEquity_bb734f8e-c2fb-40f6-985c-ebea0e2716c8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_459a3232-4051-4203-8691-bcb7d4295fb6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_459a3232-4051-4203-8691-bcb7d4295fb6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_32a654ca-e32a-428a-842b-c5f24a87e5cd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_32a654ca-e32a-428a-842b-c5f24a87e5cd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_36449746-7fe5-43c7-8750-04467097b72a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_36449746-7fe5-43c7-8750-04467097b72a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_AdjustmentsToAdditionalPaidInCapitalConversionOfConvertibleWarrants_78f01472-22c5-42c9-92d2-f78c173769f5" xlink:href="tdup-20211231.xsd#tdup_AdjustmentsToAdditionalPaidInCapitalConversionOfConvertibleWarrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_tdup_AdjustmentsToAdditionalPaidInCapitalConversionOfConvertibleWarrants_78f01472-22c5-42c9-92d2-f78c173769f5" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_412e34f5-184b-4fd0-a6ea-102d6d15cec0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_412e34f5-184b-4fd0-a6ea-102d6d15cec0" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities_1e2033d5-6cde-4830-b949-824e99ef09ef" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities_1e2033d5-6cde-4830-b949-824e99ef09ef" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_7f106ea5-e6ff-4a22-be50-1c58275497fd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_7f106ea5-e6ff-4a22-be50-1c58275497fd" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_fe1a2759-4d8b-4789-b8d7-7ae0374eaa03" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_fe1a2759-4d8b-4789-b8d7-7ae0374eaa03" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_StockIssuedDuringPeriodSharesWarrantsIssued_bf760220-e026-45b2-9d6a-cc2f9bee17b5" xlink:href="tdup-20211231.xsd#tdup_StockIssuedDuringPeriodSharesWarrantsIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_tdup_StockIssuedDuringPeriodSharesWarrantsIssued_bf760220-e026-45b2-9d6a-cc2f9bee17b5" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_f3a13ce7-cf46-4a2a-a2fc-ca6cbf5b40c5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_f3a13ce7-cf46-4a2a-a2fc-ca6cbf5b40c5" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_084ed285-adeb-493b-b53b-d01f4c0fbca9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_084ed285-adeb-493b-b53b-d01f4c0fbca9" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_9dcd1c6b-acfc-4f3c-b962-89d317be4bb8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_9dcd1c6b-acfc-4f3c-b962-89d317be4bb8" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_c9111be7-202c-4600-8e35-285bad2fcdcf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_c9111be7-202c-4600-8e35-285bad2fcdcf" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_2549dc5f-126c-4764-842b-797bb32f9ab7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_2549dc5f-126c-4764-842b-797bb32f9ab7" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_AdjustmentsToAdditionalPaidInCapitalConsiderationDelayedShareIssuance_66ee9310-959b-429e-81f8-646b7b68ec0c" xlink:href="tdup-20211231.xsd#tdup_AdjustmentsToAdditionalPaidInCapitalConsiderationDelayedShareIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_tdup_AdjustmentsToAdditionalPaidInCapitalConsiderationDelayedShareIssuance_66ee9310-959b-429e-81f8-646b7b68ec0c" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_32576650-a35a-401c-928d-142dd8ff1495" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_32576650-a35a-401c-928d-142dd8ff1495" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_26ab969f-5919-481e-9f9c-931424540e97" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_26ab969f-5919-481e-9f9c-931424540e97" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_c8677c0f-7279-4978-8ce3-e9e1f30d9b44" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_9887288f-5682-4264-8786-028dc486ffe6" xlink:to="loc_us-gaap_NetIncomeLoss_c8677c0f-7279-4978-8ce3-e9e1f30d9b44" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_3a29c9e0-f61a-43c5-b934-b77e1be1581e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_40ee57da-db97-428e-acd1-77420bd30f90" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountingStandardsUpdateExtensibleList_62636e7c-85e2-488c-83eb-58c3ef9fb4ed" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccountingStandardsUpdateExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f79a8ecb-00bd-448e-b40c-fe6851662c35" xlink:to="loc_us-gaap_AccountingStandardsUpdateExtensibleList_62636e7c-85e2-488c-83eb-58c3ef9fb4ed" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_e05f0f4b-fab2-4114-8307-d35105c334e1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_f79a8ecb-00bd-448e-b40c-fe6851662c35" xlink:to="loc_us-gaap_StatementTable_e05f0f4b-fab2-4114-8307-d35105c334e1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_51e5f2de-65ad-4c55-884e-6a021dc930b2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_e05f0f4b-fab2-4114-8307-d35105c334e1" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_51e5f2de-65ad-4c55-884e-6a021dc930b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_51e5f2de-65ad-4c55-884e-6a021dc930b2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_51e5f2de-65ad-4c55-884e-6a021dc930b2" xlink:to="loc_us-gaap_EquityComponentDomain_51e5f2de-65ad-4c55-884e-6a021dc930b2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_da63e954-87f2-4093-8ab9-f5c6232da48a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_51e5f2de-65ad-4c55-884e-6a021dc930b2" xlink:to="loc_us-gaap_EquityComponentDomain_da63e954-87f2-4093-8ab9-f5c6232da48a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_7d283c3c-9078-4869-b891-9ff6b869c004" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_da63e954-87f2-4093-8ab9-f5c6232da48a" xlink:to="loc_us-gaap_CommonStockMember_7d283c3c-9078-4869-b891-9ff6b869c004" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_793046c1-c319-4fce-9a28-d1f45d222035" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_da63e954-87f2-4093-8ab9-f5c6232da48a" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_793046c1-c319-4fce-9a28-d1f45d222035" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_32525f9c-f6c1-4f53-b064-1c11ad772616" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_da63e954-87f2-4093-8ab9-f5c6232da48a" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_32525f9c-f6c1-4f53-b064-1c11ad772616" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_e6f5c478-4680-48a7-befa-c6f1451e69dd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_da63e954-87f2-4093-8ab9-f5c6232da48a" xlink:to="loc_us-gaap_RetainedEarningsMember_e6f5c478-4680-48a7-befa-c6f1451e69dd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_269563ca-aaf4-4f85-9e35-a6b15f4b1b03" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_e05f0f4b-fab2-4114-8307-d35105c334e1" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_269563ca-aaf4-4f85-9e35-a6b15f4b1b03" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_269563ca-aaf4-4f85-9e35-a6b15f4b1b03_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_269563ca-aaf4-4f85-9e35-a6b15f4b1b03" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_269563ca-aaf4-4f85-9e35-a6b15f4b1b03_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_3399d434-8471-40fb-9c8a-a3c12dab586e" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_269563ca-aaf4-4f85-9e35-a6b15f4b1b03" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_3399d434-8471-40fb-9c8a-a3c12dab586e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_768c2754-fc31-4f2b-8ca5-46e1492df781" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_3399d434-8471-40fb-9c8a-a3c12dab586e" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_768c2754-fc31-4f2b-8ca5-46e1492df781" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_d8d618ee-b3cb-40d8-9461-b8da8ebab1f6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_e05f0f4b-fab2-4114-8307-d35105c334e1" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_d8d618ee-b3cb-40d8-9461-b8da8ebab1f6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_d8d618ee-b3cb-40d8-9461-b8da8ebab1f6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_d8d618ee-b3cb-40d8-9461-b8da8ebab1f6" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_d8d618ee-b3cb-40d8-9461-b8da8ebab1f6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_9e0bda7c-afe1-4ba0-84ce-960221a90845" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_d8d618ee-b3cb-40d8-9461-b8da8ebab1f6" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_9e0bda7c-afe1-4ba0-84ce-960221a90845" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IPOMember_5a841330-fbc3-4dcd-af1e-cb9b0573a4f3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IPOMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_9e0bda7c-afe1-4ba0-84ce-960221a90845" xlink:to="loc_us-gaap_IPOMember_5a841330-fbc3-4dcd-af1e-cb9b0573a4f3" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/OrganizationandDescriptionofBusinessDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#OrganizationandDescriptionofBusinessDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/OrganizationandDescriptionofBusinessDetails" xlink:type="extended" id="i9af80c1f373d478d92753deb7434aa0a_OrganizationandDescriptionofBusinessDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_f9512c7c-0823-4356-bf53-0eaddc4e91a6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_4b572d54-028f-4a1d-831c-8f13944d557d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f9512c7c-0823-4356-bf53-0eaddc4e91a6" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_4b572d54-028f-4a1d-831c-8f13944d557d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_229cb28d-69ff-45b0-9dff-3b4910e4c4d6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f9512c7c-0823-4356-bf53-0eaddc4e91a6" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_229cb28d-69ff-45b0-9dff-3b4910e4c4d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_04018385-4419-4b2e-b458-22c508aa15de" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f9512c7c-0823-4356-bf53-0eaddc4e91a6" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_04018385-4419-4b2e-b458-22c508aa15de" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts_4cb76853-73f7-4490-a83e-fd91c7c76d31" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f9512c7c-0823-4356-bf53-0eaddc4e91a6" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts_4cb76853-73f7-4490-a83e-fd91c7c76d31" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_2e8d82af-d200-4f1c-bf44-ae24d1e2c0ab" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f9512c7c-0823-4356-bf53-0eaddc4e91a6" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_2e8d82af-d200-4f1c-bf44-ae24d1e2c0ab" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesAuthorized_a4bd9660-208b-4ddc-b90d-0b3e3229ee8e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TemporaryEquitySharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f9512c7c-0823-4356-bf53-0eaddc4e91a6" xlink:to="loc_us-gaap_TemporaryEquitySharesAuthorized_a4bd9660-208b-4ddc-b90d-0b3e3229ee8e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_f035671b-b46e-46ac-b8c2-f395fc545196" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f9512c7c-0823-4356-bf53-0eaddc4e91a6" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_f035671b-b46e-46ac-b8c2-f395fc545196" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ReclassificationOfCommonStockBetweenClassesShares_f6dea2f9-6c2f-4d62-a445-0109f3686913" xlink:href="tdup-20211231.xsd#tdup_ReclassificationOfCommonStockBetweenClassesShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f9512c7c-0823-4356-bf53-0eaddc4e91a6" xlink:to="loc_tdup_ReclassificationOfCommonStockBetweenClassesShares_f6dea2f9-6c2f-4d62-a445-0109f3686913" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_81ab1a53-5d2e-43c9-95fb-527fc87ac374" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfWarrantOrRightOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f9512c7c-0823-4356-bf53-0eaddc4e91a6" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_81ab1a53-5d2e-43c9-95fb-527fc87ac374" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfStockIssuanceCosts_02ee2583-dbbd-4872-904d-3cdad46b1c5b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsOfStockIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_f9512c7c-0823-4356-bf53-0eaddc4e91a6" xlink:to="loc_us-gaap_PaymentsOfStockIssuanceCosts_02ee2583-dbbd-4872-904d-3cdad46b1c5b" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_0ed20e36-d64a-40be-8d7b-7eb277613079" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_f9512c7c-0823-4356-bf53-0eaddc4e91a6" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_0ed20e36-d64a-40be-8d7b-7eb277613079" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_a9528524-0832-4bd0-abe9-f743b10660e6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_0ed20e36-d64a-40be-8d7b-7eb277613079" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_a9528524-0832-4bd0-abe9-f743b10660e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_a9528524-0832-4bd0-abe9-f743b10660e6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_a9528524-0832-4bd0-abe9-f743b10660e6" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_a9528524-0832-4bd0-abe9-f743b10660e6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_5bf0d734-6b6f-4da5-98c0-d66d5ba172eb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_a9528524-0832-4bd0-abe9-f743b10660e6" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_5bf0d734-6b6f-4da5-98c0-d66d5ba172eb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IPOMember_10f2332b-b33e-4130-9c78-3acff1eb6c94" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IPOMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_5bf0d734-6b6f-4da5-98c0-d66d5ba172eb" xlink:to="loc_us-gaap_IPOMember_10f2332b-b33e-4130-9c78-3acff1eb6c94" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_dd986f9c-9686-498c-ba9a-c6e68ab031f0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_0ed20e36-d64a-40be-8d7b-7eb277613079" xlink:to="loc_us-gaap_StatementClassOfStockAxis_dd986f9c-9686-498c-ba9a-c6e68ab031f0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_dd986f9c-9686-498c-ba9a-c6e68ab031f0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_dd986f9c-9686-498c-ba9a-c6e68ab031f0" xlink:to="loc_us-gaap_ClassOfStockDomain_dd986f9c-9686-498c-ba9a-c6e68ab031f0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_8851ee2f-1646-42e0-ac13-33a604ba7d43" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_dd986f9c-9686-498c-ba9a-c6e68ab031f0" xlink:to="loc_us-gaap_ClassOfStockDomain_8851ee2f-1646-42e0-ac13-33a604ba7d43" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_3bac78f7-41f8-4a33-ad05-e6b9eea70190" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_8851ee2f-1646-42e0-ac13-33a604ba7d43" xlink:to="loc_us-gaap_CommonClassAMember_3bac78f7-41f8-4a33-ad05-e6b9eea70190" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_9fb4d89d-18c3-478b-b280-82f7d65cb38c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_8851ee2f-1646-42e0-ac13-33a604ba7d43" xlink:to="loc_us-gaap_CommonClassBMember_9fb4d89d-18c3-478b-b280-82f7d65cb38c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_2b9ef6e1-1a67-4156-b5b2-f38542665ffd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_0ed20e36-d64a-40be-8d7b-7eb277613079" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_2b9ef6e1-1a67-4156-b5b2-f38542665ffd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_2b9ef6e1-1a67-4156-b5b2-f38542665ffd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_2b9ef6e1-1a67-4156-b5b2-f38542665ffd" xlink:to="loc_us-gaap_EquityComponentDomain_2b9ef6e1-1a67-4156-b5b2-f38542665ffd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_21b4dff4-5bd6-47f9-82a3-93984d7164f6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_2b9ef6e1-1a67-4156-b5b2-f38542665ffd" xlink:to="loc_us-gaap_EquityComponentDomain_21b4dff4-5bd6-47f9-82a3-93984d7164f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_368711f6-56d9-4706-b992-43449c52d1aa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_21b4dff4-5bd6-47f9-82a3-93984d7164f6" xlink:to="loc_us-gaap_CommonStockMember_368711f6-56d9-4706-b992-43449c52d1aa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_5cbeac08-f3e6-4387-b0cf-b3e32885caac" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_0ed20e36-d64a-40be-8d7b-7eb277613079" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_5cbeac08-f3e6-4387-b0cf-b3e32885caac" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_5cbeac08-f3e6-4387-b0cf-b3e32885caac_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_5cbeac08-f3e6-4387-b0cf-b3e32885caac" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_5cbeac08-f3e6-4387-b0cf-b3e32885caac_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_7b067d35-f038-4421-8f5b-a3f7b4542268" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_5cbeac08-f3e6-4387-b0cf-b3e32885caac" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_7b067d35-f038-4421-8f5b-a3f7b4542268" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ClassBCommonStockWarrantMember_7e804960-03fe-4f6e-b724-67f763e43cfb" xlink:href="tdup-20211231.xsd#tdup_ClassBCommonStockWarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_7b067d35-f038-4421-8f5b-a3f7b4542268" xlink:to="loc_tdup_ClassBCommonStockWarrantMember_7e804960-03fe-4f6e-b724-67f763e43cfb" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/SignificantAccountingPoliciesNarrativeDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#SignificantAccountingPoliciesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/SignificantAccountingPoliciesNarrativeDetails" xlink:type="extended" id="i7aef163558274062a836ccd5cb312ca1_SignificantAccountingPoliciesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_RevenueFromContractWithCustomerReturnPeriod_489d4fdd-de89-4e9a-8de5-d93bef5e8588" xlink:href="tdup-20211231.xsd#tdup_RevenueFromContractWithCustomerReturnPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:to="loc_tdup_RevenueFromContractWithCustomerReturnPeriod_489d4fdd-de89-4e9a-8de5-d93bef5e8588" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_f90bce99-2099-48a7-a87f-cb1596757c4f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_f90bce99-2099-48a7-a87f-cb1596757c4f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_0310682f-d70c-4e51-bafb-a0ed532a1abb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerLiabilityRevenueRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_0310682f-d70c-4e51-bafb-a0ed532a1abb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ContractWithCustomerLiabilityRevenueRecognizedIncludingCurrentPeriod_1407237a-5da4-4849-965c-6c0cdabcaf7c" xlink:href="tdup-20211231.xsd#tdup_ContractWithCustomerLiabilityRevenueRecognizedIncludingCurrentPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:to="loc_tdup_ContractWithCustomerLiabilityRevenueRecognizedIncludingCurrentPeriod_1407237a-5da4-4849-965c-6c0cdabcaf7c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_RevenueFromContractWithCustomerRecognitionPeriod_bfcb9909-4185-4894-8508-f4942c3dc876" xlink:href="tdup-20211231.xsd#tdup_RevenueFromContractWithCustomerRecognitionPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:to="loc_tdup_RevenueFromContractWithCustomerRecognitionPeriod_bfcb9909-4185-4894-8508-f4942c3dc876" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_RevenueFromContractWithCustomerLoyaltyPointExpirationPeriod_5dc99d52-b89f-4dc6-ac06-027a27b2f09c" xlink:href="tdup-20211231.xsd#tdup_RevenueFromContractWithCustomerLoyaltyPointExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:to="loc_tdup_RevenueFromContractWithCustomerLoyaltyPointExpirationPeriod_5dc99d52-b89f-4dc6-ac06-027a27b2f09c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfOperatingSegments_43d5931a-bdf5-454d-a0a8-75fd9f617881" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NumberOfOperatingSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:to="loc_us-gaap_NumberOfOperatingSegments_43d5931a-bdf5-454d-a0a8-75fd9f617881" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_74d4521c-d566-49fe-bc78-2c1af8573b41" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NumberOfReportableSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:to="loc_us-gaap_NumberOfReportableSegments_74d4521c-d566-49fe-bc78-2c1af8573b41" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_0eb96883-4d7d-40de-b67d-77293d31d6bc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_0eb96883-4d7d-40de-b67d-77293d31d6bc" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OciBeforeReclassificationsNetOfTaxAttributableToParent_6c958356-1611-4cda-baed-880d53b646ee" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OciBeforeReclassificationsNetOfTaxAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:to="loc_us-gaap_OciBeforeReclassificationsNetOfTaxAttributableToParent_6c958356-1611-4cda-baed-880d53b646ee" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashNoncurrent_5a249f47-6783-4b3c-b9db-6db85926983b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedCashNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:to="loc_us-gaap_RestrictedCashNoncurrent_5a249f47-6783-4b3c-b9db-6db85926983b" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TangibleAssetImpairmentCharges_1564cdfa-bdc8-44d6-8e62-a4948c818a39" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TangibleAssetImpairmentCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:to="loc_us-gaap_TangibleAssetImpairmentCharges_1564cdfa-bdc8-44d6-8e62-a4948c818a39" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_513fc71d-6d76-41f5-ae59-5133da00a1bd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:to="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_513fc71d-6d76-41f5-ae59-5133da00a1bd" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent_b1209c5f-528d-42ed-b2e7-f4c93f2d7ca1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:to="loc_us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent_b1209c5f-528d-42ed-b2e7-f4c93f2d7ca1" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLossCumulativeAmount_39ab443d-1d84-49f3-aa7d-55284dcd9fbf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLossCumulativeAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:to="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLossCumulativeAmount_39ab443d-1d84-49f3-aa7d-55284dcd9fbf" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ContractWithCustomerLiabilityCurrentConversionPeriod_0b0cecfd-b4a1-4ff1-ae49-15daf7739e18" xlink:href="tdup-20211231.xsd#tdup_ContractWithCustomerLiabilityCurrentConversionPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:to="loc_tdup_ContractWithCustomerLiabilityCurrentConversionPeriod_0b0cecfd-b4a1-4ff1-ae49-15daf7739e18" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_b8bc8ffa-cf2d-4d3e-a7f1-880867f34f7e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_b8bc8ffa-cf2d-4d3e-a7f1-880867f34f7e" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdvertisingExpense_e1a935e7-78c8-4e8b-959c-3db446c4285c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdvertisingExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:to="loc_us-gaap_AdvertisingExpense_e1a935e7-78c8-4e8b-959c-3db446c4285c" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_4577ed94-a217-47e4-bea6-11bcd61e5637" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_777c8f8d-2cbc-4aa1-a075-4539e0b41e22" xlink:to="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_4577ed94-a217-47e4-bea6-11bcd61e5637" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_3cc7e476-e9ac-402d-88fd-74946492c845" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_4577ed94-a217-47e4-bea6-11bcd61e5637" xlink:to="loc_srt_ProductOrServiceAxis_3cc7e476-e9ac-402d-88fd-74946492c845" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_3cc7e476-e9ac-402d-88fd-74946492c845_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_3cc7e476-e9ac-402d-88fd-74946492c845" xlink:to="loc_srt_ProductsAndServicesDomain_3cc7e476-e9ac-402d-88fd-74946492c845_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_05dace8f-2790-490a-b57d-4ffa1e8a2457" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_3cc7e476-e9ac-402d-88fd-74946492c845" xlink:to="loc_srt_ProductsAndServicesDomain_05dace8f-2790-490a-b57d-4ffa1e8a2457" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_LoyaltyProgramMember_05bf6f87-c8ed-4bb4-bd1f-0257dd90f12c" xlink:href="tdup-20211231.xsd#tdup_LoyaltyProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_05dace8f-2790-490a-b57d-4ffa1e8a2457" xlink:to="loc_tdup_LoyaltyProgramMember_05bf6f87-c8ed-4bb4-bd1f-0257dd90f12c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_GiftCardMember_4f696238-344e-4ce6-8b79-d3d6f0922905" xlink:href="tdup-20211231.xsd#tdup_GiftCardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_05dace8f-2790-490a-b57d-4ffa1e8a2457" xlink:to="loc_tdup_GiftCardMember_4f696238-344e-4ce6-8b79-d3d6f0922905" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SiteCreditMember_5c08b80f-de36-40ed-b3c4-01839d04558d" xlink:href="tdup-20211231.xsd#tdup_SiteCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_05dace8f-2790-490a-b57d-4ffa1e8a2457" xlink:to="loc_tdup_SiteCreditMember_5c08b80f-de36-40ed-b3c4-01839d04558d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_4382f098-76c0-4dce-b08d-adfe8de755b1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_4577ed94-a217-47e4-bea6-11bcd61e5637" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_4382f098-76c0-4dce-b08d-adfe8de755b1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_4382f098-76c0-4dce-b08d-adfe8de755b1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_4382f098-76c0-4dce-b08d-adfe8de755b1" xlink:to="loc_us-gaap_EquityComponentDomain_4382f098-76c0-4dce-b08d-adfe8de755b1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_4644deef-7d2e-4bc0-ab79-fe38bb954ec9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_4382f098-76c0-4dce-b08d-adfe8de755b1" xlink:to="loc_us-gaap_EquityComponentDomain_4644deef-7d2e-4bc0-ab79-fe38bb954ec9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_d1b2bbcb-82c6-4252-80a1-918e27162990" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_4644deef-7d2e-4bc0-ab79-fe38bb954ec9" xlink:to="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_d1b2bbcb-82c6-4252-80a1-918e27162990" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_a66a451a-dc83-45c6-8e06-9b7a259fcbd3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TimingOfTransferOfGoodOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_4577ed94-a217-47e4-bea6-11bcd61e5637" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_a66a451a-dc83-45c6-8e06-9b7a259fcbd3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_a66a451a-dc83-45c6-8e06-9b7a259fcbd3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_a66a451a-dc83-45c6-8e06-9b7a259fcbd3" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_a66a451a-dc83-45c6-8e06-9b7a259fcbd3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_aa5529e5-7f54-4d1a-a576-626fe3a030bc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_a66a451a-dc83-45c6-8e06-9b7a259fcbd3" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_aa5529e5-7f54-4d1a-a576-626fe3a030bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredAtPointInTimeMember_626c889c-9559-4905-bb33-7ef5f615a8cc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TransferredAtPointInTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_aa5529e5-7f54-4d1a-a576-626fe3a030bc" xlink:to="loc_us-gaap_TransferredAtPointInTimeMember_626c889c-9559-4905-bb33-7ef5f615a8cc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredOverTimeMember_5f3b449e-7a23-4de2-b10e-3d21762dd09c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TransferredOverTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_aa5529e5-7f54-4d1a-a576-626fe3a030bc" xlink:to="loc_us-gaap_TransferredOverTimeMember_5f3b449e-7a23-4de2-b10e-3d21762dd09c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_05744277-1b80-41a4-8691-a9f0460df20e" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_4577ed94-a217-47e4-bea6-11bcd61e5637" xlink:to="loc_srt_RangeAxis_05744277-1b80-41a4-8691-a9f0460df20e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_05744277-1b80-41a4-8691-a9f0460df20e_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_05744277-1b80-41a4-8691-a9f0460df20e" xlink:to="loc_srt_RangeMember_05744277-1b80-41a4-8691-a9f0460df20e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_2ae1ab27-baa7-4254-a7ec-07078a0f0d06" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_05744277-1b80-41a4-8691-a9f0460df20e" xlink:to="loc_srt_RangeMember_2ae1ab27-baa7-4254-a7ec-07078a0f0d06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_600c5b2e-b05c-4d4f-921f-1e164fc93388" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_2ae1ab27-baa7-4254-a7ec-07078a0f0d06" xlink:to="loc_srt_MinimumMember_600c5b2e-b05c-4d4f-921f-1e164fc93388" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_9dd1fc5c-253a-46b3-9553-c41176cecd75" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_2ae1ab27-baa7-4254-a7ec-07078a0f0d06" xlink:to="loc_srt_MaximumMember_9dd1fc5c-253a-46b3-9553-c41176cecd75" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_09630809-5c43-495a-a270-a9045108f7c0" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_4577ed94-a217-47e4-bea6-11bcd61e5637" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_09630809-5c43-495a-a270-a9045108f7c0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_09630809-5c43-495a-a270-a9045108f7c0_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_09630809-5c43-495a-a270-a9045108f7c0" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_09630809-5c43-495a-a270-a9045108f7c0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_91b8009e-c14b-48e6-a8bd-be86e375da5c" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_09630809-5c43-495a-a270-a9045108f7c0" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_91b8009e-c14b-48e6-a8bd-be86e375da5c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_VoperoIncMember_fb707837-3c5d-4caf-a550-5bec46c367c3" xlink:href="tdup-20211231.xsd#tdup_VoperoIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_91b8009e-c14b-48e6-a8bd-be86e375da5c" xlink:to="loc_tdup_VoperoIncMember_fb707837-3c5d-4caf-a550-5bec46c367c3" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/SignificantAccountingPoliciesRevenuebyGeographicAreaDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#SignificantAccountingPoliciesRevenuebyGeographicAreaDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/SignificantAccountingPoliciesRevenuebyGeographicAreaDetails" xlink:type="extended" id="i7a940cd8fd5e427ca5430aca750fb358_SignificantAccountingPoliciesRevenuebyGeographicAreaDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_03a499c5-07aa-45b0-82aa-13b2460a769b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_e3d9654c-a93b-460c-a5a9-ee637b0d960e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_03a499c5-07aa-45b0-82aa-13b2460a769b" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_e3d9654c-a93b-460c-a5a9-ee637b0d960e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_18a47511-1ee1-4f25-a09d-672cd5d6d804" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_03a499c5-07aa-45b0-82aa-13b2460a769b" xlink:to="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_18a47511-1ee1-4f25-a09d-672cd5d6d804" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_cf4f7c32-8cfa-4238-8d35-fc410dffe1e1" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_18a47511-1ee1-4f25-a09d-672cd5d6d804" xlink:to="loc_srt_StatementGeographicalAxis_cf4f7c32-8cfa-4238-8d35-fc410dffe1e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_cf4f7c32-8cfa-4238-8d35-fc410dffe1e1_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_cf4f7c32-8cfa-4238-8d35-fc410dffe1e1" xlink:to="loc_srt_SegmentGeographicalDomain_cf4f7c32-8cfa-4238-8d35-fc410dffe1e1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_2a6678d8-3200-41b7-9e72-91aa8393b303" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_cf4f7c32-8cfa-4238-8d35-fc410dffe1e1" xlink:to="loc_srt_SegmentGeographicalDomain_2a6678d8-3200-41b7-9e72-91aa8393b303" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NorthAmericaMember_cddaf6fb-1159-4c7f-9038-14cacffdf2cb" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_NorthAmericaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_2a6678d8-3200-41b7-9e72-91aa8393b303" xlink:to="loc_srt_NorthAmericaMember_cddaf6fb-1159-4c7f-9038-14cacffdf2cb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EuropeMember_8006237d-8fd2-4984-87b5-2070c65d5bcf" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_EuropeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_2a6678d8-3200-41b7-9e72-91aa8393b303" xlink:to="loc_srt_EuropeMember_8006237d-8fd2-4984-87b5-2070c65d5bcf" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/SignificantAccountingPoliciesScheduleofPropertyandEquipmentNetDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#SignificantAccountingPoliciesScheduleofPropertyandEquipmentNetDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/SignificantAccountingPoliciesScheduleofPropertyandEquipmentNetDetails" xlink:type="extended" id="i8232a84e66904233b209a5cd1591f877_SignificantAccountingPoliciesScheduleofPropertyandEquipmentNetDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_597f127c-3505-4e85-a2b1-64cf1e5ccbcf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_6058a066-2445-4883-81ea-bb09a9730022" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_597f127c-3505-4e85-a2b1-64cf1e5ccbcf" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_6058a066-2445-4883-81ea-bb09a9730022" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_1f6d8352-94ed-4bbc-8b47-1892d5a82907" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_597f127c-3505-4e85-a2b1-64cf1e5ccbcf" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_1f6d8352-94ed-4bbc-8b47-1892d5a82907" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_6f3c6ce9-6e50-4779-b41c-2670b36bbc98" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_1f6d8352-94ed-4bbc-8b47-1892d5a82907" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_6f3c6ce9-6e50-4779-b41c-2670b36bbc98" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_6f3c6ce9-6e50-4779-b41c-2670b36bbc98_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_6f3c6ce9-6e50-4779-b41c-2670b36bbc98" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_6f3c6ce9-6e50-4779-b41c-2670b36bbc98_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_66bce1eb-ac62-4e45-b6cf-2018b8d0e95c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_6f3c6ce9-6e50-4779-b41c-2670b36bbc98" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_66bce1eb-ac62-4e45-b6cf-2018b8d0e95c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MachineryAndEquipmentMember_ab352753-4889-44ec-9172-1088ad9be8fa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_MachineryAndEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_66bce1eb-ac62-4e45-b6cf-2018b8d0e95c" xlink:to="loc_us-gaap_MachineryAndEquipmentMember_ab352753-4889-44ec-9172-1088ad9be8fa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareDevelopmentMember_92e72aad-ac36-434a-89e4-8f9186bb4e36" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SoftwareDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_66bce1eb-ac62-4e45-b6cf-2018b8d0e95c" xlink:to="loc_us-gaap_SoftwareDevelopmentMember_92e72aad-ac36-434a-89e4-8f9186bb4e36" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ComputerEquipmentAndSoftwareMember_96bda1aa-3521-4068-ac23-f444c654c08a" xlink:href="tdup-20211231.xsd#tdup_ComputerEquipmentAndSoftwareMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_66bce1eb-ac62-4e45-b6cf-2018b8d0e95c" xlink:to="loc_tdup_ComputerEquipmentAndSoftwareMember_96bda1aa-3521-4068-ac23-f444c654c08a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_dd9e709a-11cb-42fa-b594-125a3c3ecc23" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_66bce1eb-ac62-4e45-b6cf-2018b8d0e95c" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_dd9e709a-11cb-42fa-b594-125a3c3ecc23" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_353e83d4-11a4-4529-86b3-12ba8b830be3" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_1f6d8352-94ed-4bbc-8b47-1892d5a82907" xlink:to="loc_srt_RangeAxis_353e83d4-11a4-4529-86b3-12ba8b830be3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_353e83d4-11a4-4529-86b3-12ba8b830be3_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_353e83d4-11a4-4529-86b3-12ba8b830be3" xlink:to="loc_srt_RangeMember_353e83d4-11a4-4529-86b3-12ba8b830be3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_b2f30d7b-d806-4cd6-bbf1-49e35196d8ce" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_353e83d4-11a4-4529-86b3-12ba8b830be3" xlink:to="loc_srt_RangeMember_b2f30d7b-d806-4cd6-bbf1-49e35196d8ce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_d59646a8-01a6-4fc6-baf5-6b4421f06107" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_b2f30d7b-d806-4cd6-bbf1-49e35196d8ce" xlink:to="loc_srt_MinimumMember_d59646a8-01a6-4fc6-baf5-6b4421f06107" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_d14354cd-8f9a-45cb-aa54-cc72bced7b95" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_b2f30d7b-d806-4cd6-bbf1-49e35196d8ce" xlink:to="loc_srt_MaximumMember_d14354cd-8f9a-45cb-aa54-cc72bced7b95" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofFinancialInstrumentsMeasuredatFairValueDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#FinancialInstrumentsandFairValueMeasurementsScheduleofFinancialInstrumentsMeasuredatFairValueDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofFinancialInstrumentsMeasuredatFairValueDetails" xlink:type="extended" id="i3d5972eafcea4bd99e500e7a0492b261_FinancialInstrumentsandFairValueMeasurementsScheduleofFinancialInstrumentsMeasuredatFairValueDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_8d67ca4d-45e8-4f6b-85bb-a7aca51ef250" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosureAbstract_5b7ef233-627d-4717-9f0b-1dd7735ec970" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AssetsFairValueDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_8d67ca4d-45e8-4f6b-85bb-a7aca51ef250" xlink:to="loc_us-gaap_AssetsFairValueDisclosureAbstract_5b7ef233-627d-4717-9f0b-1dd7735ec970" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_cda676f3-6992-480a-a74e-a12383a3fe2f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_5b7ef233-627d-4717-9f0b-1dd7735ec970" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_cda676f3-6992-480a-a74e-a12383a3fe2f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_7dcfe9db-1387-4f7f-80dd-158195085ec1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_5b7ef233-627d-4717-9f0b-1dd7735ec970" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_7dcfe9db-1387-4f7f-80dd-158195085ec1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_1c31b84e-d083-4b1e-b27a-5c0bc0c1d1c0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_5b7ef233-627d-4717-9f0b-1dd7735ec970" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_1c31b84e-d083-4b1e-b27a-5c0bc0c1d1c0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAbstract_25146b61-d4e9-49ee-bd5a-226fe2b9b83c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CashAndCashEquivalentsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_8d67ca4d-45e8-4f6b-85bb-a7aca51ef250" xlink:to="loc_us-gaap_CashAndCashEquivalentsAbstract_25146b61-d4e9-49ee-bd5a-226fe2b9b83c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_ae9777fc-81c1-42f1-bb45-a8235e7a94ef" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsAbstract_25146b61-d4e9-49ee-bd5a-226fe2b9b83c" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_ae9777fc-81c1-42f1-bb45-a8235e7a94ef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstanding_bacf7623-2b93-4f39-9f06-8e1c9fabd2a6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_WarrantsAndRightsOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_8d67ca4d-45e8-4f6b-85bb-a7aca51ef250" xlink:to="loc_us-gaap_WarrantsAndRightsOutstanding_bacf7623-2b93-4f39-9f06-8e1c9fabd2a6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_faca08a1-46fe-4aba-98b7-335c8089f482" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_8d67ca4d-45e8-4f6b-85bb-a7aca51ef250" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_faca08a1-46fe-4aba-98b7-335c8089f482" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_14dbd58b-fcb3-491c-8809-2ec551494108" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_faca08a1-46fe-4aba-98b7-335c8089f482" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_14dbd58b-fcb3-491c-8809-2ec551494108" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_14dbd58b-fcb3-491c-8809-2ec551494108_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_14dbd58b-fcb3-491c-8809-2ec551494108" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_14dbd58b-fcb3-491c-8809-2ec551494108_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_4f22c8f7-0ff3-4f6f-bed8-dee8c17139f0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_14dbd58b-fcb3-491c-8809-2ec551494108" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_4f22c8f7-0ff3-4f6f-bed8-dee8c17139f0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_571f4cc2-4381-48f7-a48c-30866281f356" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_4f22c8f7-0ff3-4f6f-bed8-dee8c17139f0" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_571f4cc2-4381-48f7-a48c-30866281f356" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_c5e02164-69c5-4ec3-837f-4edf14affd0a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_faca08a1-46fe-4aba-98b7-335c8089f482" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_c5e02164-69c5-4ec3-837f-4edf14affd0a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_c5e02164-69c5-4ec3-837f-4edf14affd0a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_c5e02164-69c5-4ec3-837f-4edf14affd0a" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_c5e02164-69c5-4ec3-837f-4edf14affd0a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_7e9948ff-060e-43a5-ad9b-45dfc0f9c84b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_c5e02164-69c5-4ec3-837f-4edf14affd0a" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_7e9948ff-060e-43a5-ad9b-45dfc0f9c84b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_1570cab2-cc0a-4688-811f-c8f147c2b1c2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_7e9948ff-060e-43a5-ad9b-45dfc0f9c84b" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_1570cab2-cc0a-4688-811f-c8f147c2b1c2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_c23eecfe-7426-45e4-bfca-da2b9ca3c9c6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_7e9948ff-060e-43a5-ad9b-45dfc0f9c84b" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_c23eecfe-7426-45e4-bfca-da2b9ca3c9c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_99c95b2c-57e0-4c2e-8b68-855c65f50b69" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_7e9948ff-060e-43a5-ad9b-45dfc0f9c84b" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_99c95b2c-57e0-4c2e-8b68-855c65f50b69" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_abf2cec6-40a6-4947-a373-712df3bfde24" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_faca08a1-46fe-4aba-98b7-335c8089f482" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_abf2cec6-40a6-4947-a373-712df3bfde24" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_abf2cec6-40a6-4947-a373-712df3bfde24_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_abf2cec6-40a6-4947-a373-712df3bfde24" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_abf2cec6-40a6-4947-a373-712df3bfde24_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_925a55dc-6429-45c1-bb5e-ee83a9ba3d41" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_abf2cec6-40a6-4947-a373-712df3bfde24" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_925a55dc-6429-45c1-bb5e-ee83a9ba3d41" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_77040738-a8f5-479f-84e7-f9aac85e953f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_925a55dc-6429-45c1-bb5e-ee83a9ba3d41" xlink:to="loc_us-gaap_MoneyMarketFundsMember_77040738-a8f5-479f-84e7-f9aac85e953f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_f0b21f5a-fd72-4792-a43c-e1a34841c68a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_925a55dc-6429-45c1-bb5e-ee83a9ba3d41" xlink:to="loc_us-gaap_CommercialPaperMember_f0b21f5a-fd72-4792-a43c-e1a34841c68a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_87bfb600-9cb6-4c16-85e7-6fdfa8885294" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_faca08a1-46fe-4aba-98b7-335c8089f482" xlink:to="loc_us-gaap_FinancialInstrumentAxis_87bfb600-9cb6-4c16-85e7-6fdfa8885294" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_87bfb600-9cb6-4c16-85e7-6fdfa8885294_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_87bfb600-9cb6-4c16-85e7-6fdfa8885294" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_87bfb600-9cb6-4c16-85e7-6fdfa8885294_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_a131e13a-0655-467b-a70d-949d25bb607f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_87bfb600-9cb6-4c16-85e7-6fdfa8885294" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_a131e13a-0655-467b-a70d-949d25bb607f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_f59227b2-b94a-4bd4-b6ae-c54461abaa6e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_USTreasurySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_a131e13a-0655-467b-a70d-949d25bb607f" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_f59227b2-b94a-4bd4-b6ae-c54461abaa6e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_bda3f8a0-cf5f-4d1b-825b-07157d183703" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_a131e13a-0655-467b-a70d-949d25bb607f" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_bda3f8a0-cf5f-4d1b-825b-07157d183703" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_51e3ef8e-15da-46a2-a0da-c716b0c4a25a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_a131e13a-0655-467b-a70d-949d25bb607f" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_51e3ef8e-15da-46a2-a0da-c716b0c4a25a" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofAssetsandLiabilitiesDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#FinancialInstrumentsandFairValueMeasurementsScheduleofAssetsandLiabilitiesDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofAssetsandLiabilitiesDetails" xlink:type="extended" id="i5f74a3b6c794446999c1316273473a97_FinancialInstrumentsandFairValueMeasurementsScheduleofAssetsandLiabilitiesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_a3b5ed42-8a82-4aff-8d45-866de1e0ef80" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_c87749ec-f849-42eb-8c1b-e9d1369638a7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_a3b5ed42-8a82-4aff-8d45-866de1e0ef80" xlink:to="loc_us-gaap_AssetsAbstract_c87749ec-f849-42eb-8c1b-e9d1369638a7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAbstract_8964657d-2abc-488a-b3cb-dcecb0c5be58" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CashAndCashEquivalentsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_c87749ec-f849-42eb-8c1b-e9d1369638a7" xlink:to="loc_us-gaap_CashAndCashEquivalentsAbstract_8964657d-2abc-488a-b3cb-dcecb0c5be58" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_9767a7a7-d02b-4454-a31c-04967b013098" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsAbstract_8964657d-2abc-488a-b3cb-dcecb0c5be58" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_9767a7a7-d02b-4454-a31c-04967b013098" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAbstract_37d1d202-50e7-4759-b1c0-2ea09f5d2e88" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LiabilitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_a3b5ed42-8a82-4aff-8d45-866de1e0ef80" xlink:to="loc_us-gaap_LiabilitiesAbstract_37d1d202-50e7-4759-b1c0-2ea09f5d2e88" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstanding_8d57fdc7-bb64-421c-920a-329509bca529" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_WarrantsAndRightsOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_37d1d202-50e7-4759-b1c0-2ea09f5d2e88" xlink:to="loc_us-gaap_WarrantsAndRightsOutstanding_8d57fdc7-bb64-421c-920a-329509bca529" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialLiabilitiesFairValueDisclosure_3e252065-e1a9-48e4-95cc-0bfdae2a9cf7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FinancialLiabilitiesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_37d1d202-50e7-4759-b1c0-2ea09f5d2e88" xlink:to="loc_us-gaap_FinancialLiabilitiesFairValueDisclosure_3e252065-e1a9-48e4-95cc-0bfdae2a9cf7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_6f9a208a-772c-453f-8471-18dabef0060b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_a3b5ed42-8a82-4aff-8d45-866de1e0ef80" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_6f9a208a-772c-453f-8471-18dabef0060b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_f8de29ca-0676-4fd3-b33f-ca8403950a1b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_6f9a208a-772c-453f-8471-18dabef0060b" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_f8de29ca-0676-4fd3-b33f-ca8403950a1b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_f8de29ca-0676-4fd3-b33f-ca8403950a1b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_f8de29ca-0676-4fd3-b33f-ca8403950a1b" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_f8de29ca-0676-4fd3-b33f-ca8403950a1b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_9b38c6e6-0703-411d-b233-0a6d6f7c1877" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_f8de29ca-0676-4fd3-b33f-ca8403950a1b" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_9b38c6e6-0703-411d-b233-0a6d6f7c1877" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_ddb225eb-eb5a-4de5-81da-95fab751aa49" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_9b38c6e6-0703-411d-b233-0a6d6f7c1877" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_ddb225eb-eb5a-4de5-81da-95fab751aa49" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_8fa65b68-1d36-47c1-ac5a-67684528c9c7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_6f9a208a-772c-453f-8471-18dabef0060b" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_8fa65b68-1d36-47c1-ac5a-67684528c9c7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_8fa65b68-1d36-47c1-ac5a-67684528c9c7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_8fa65b68-1d36-47c1-ac5a-67684528c9c7" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_8fa65b68-1d36-47c1-ac5a-67684528c9c7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_6df16dea-2585-41ee-b2ec-fa7aaf4eb98d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_8fa65b68-1d36-47c1-ac5a-67684528c9c7" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_6df16dea-2585-41ee-b2ec-fa7aaf4eb98d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_ec20cc64-13ef-44b9-9a31-ca36c63c359e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_6df16dea-2585-41ee-b2ec-fa7aaf4eb98d" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_ec20cc64-13ef-44b9-9a31-ca36c63c359e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_0aedbbfc-e072-499e-aac8-f5508d079b70" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_6df16dea-2585-41ee-b2ec-fa7aaf4eb98d" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_0aedbbfc-e072-499e-aac8-f5508d079b70" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_8ff36645-d6ec-4438-81b6-22592d940199" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_6df16dea-2585-41ee-b2ec-fa7aaf4eb98d" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_8ff36645-d6ec-4438-81b6-22592d940199" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_6409eab4-9dd7-454d-8382-3363666f8561" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_6f9a208a-772c-453f-8471-18dabef0060b" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_6409eab4-9dd7-454d-8382-3363666f8561" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_6409eab4-9dd7-454d-8382-3363666f8561_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_6409eab4-9dd7-454d-8382-3363666f8561" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_6409eab4-9dd7-454d-8382-3363666f8561_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_97e7734f-5600-4467-8b1e-9605d5b57ba7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_6409eab4-9dd7-454d-8382-3363666f8561" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_97e7734f-5600-4467-8b1e-9605d5b57ba7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_30f57836-9f20-47d4-8078-af4c043c51e3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_97e7734f-5600-4467-8b1e-9605d5b57ba7" xlink:to="loc_us-gaap_MoneyMarketFundsMember_30f57836-9f20-47d4-8078-af4c043c51e3" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/FinancialInstrumentsandFairValueMeasurementsNarrativeDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#FinancialInstrumentsandFairValueMeasurementsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/FinancialInstrumentsandFairValueMeasurementsNarrativeDetails" xlink:type="extended" id="id59dfa950e5a4cde848704cc425d6345_FinancialInstrumentsandFairValueMeasurementsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_89cc67ff-cae7-4bb2-acf2-03067403ea67" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_4c8a36d3-a876-4995-af1c-35c35650bbea" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_89cc67ff-cae7-4bb2-acf2-03067403ea67" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_4c8a36d3-a876-4995-af1c-35c35650bbea" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue_65571358-bc5d-4e68-bb5d-eb6f32f1af8a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_89cc67ff-cae7-4bb2-acf2-03067403ea67" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue_65571358-bc5d-4e68-bb5d-eb6f32f1af8a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue_6c8ef6b4-c3ef-4215-9d19-dc56025f33dd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_89cc67ff-cae7-4bb2-acf2-03067403ea67" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue_6c8ef6b4-c3ef-4215-9d19-dc56025f33dd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_e5386530-6290-498f-9685-f87881ec3c15" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_89cc67ff-cae7-4bb2-acf2-03067403ea67" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_e5386530-6290-498f-9685-f87881ec3c15" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_72b7609b-2778-4b1d-bc6b-f8d78d44c953" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_e5386530-6290-498f-9685-f87881ec3c15" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_72b7609b-2778-4b1d-bc6b-f8d78d44c953" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_72b7609b-2778-4b1d-bc6b-f8d78d44c953_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_72b7609b-2778-4b1d-bc6b-f8d78d44c953" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_72b7609b-2778-4b1d-bc6b-f8d78d44c953_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_ff3eda07-4b15-4e35-97fc-8dee8d8b46f7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_72b7609b-2778-4b1d-bc6b-f8d78d44c953" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_ff3eda07-4b15-4e35-97fc-8dee8d8b46f7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_50babd50-3496-4748-b01d-dc4cbf11f207" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_ff3eda07-4b15-4e35-97fc-8dee8d8b46f7" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_50babd50-3496-4748-b01d-dc4cbf11f207" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/PropertyandEquipmentNetDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#PropertyandEquipmentNetDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/PropertyandEquipmentNetDetails" xlink:type="extended" id="i12e66d5e4a924a1f9c2667677436b7f3_PropertyandEquipmentNetDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_464b350b-2447-47d8-a25c-ad5572aa6fd3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_f148a3cc-e418-4e3f-9ee0-c89c46c96082" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_464b350b-2447-47d8-a25c-ad5572aa6fd3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_f148a3cc-e418-4e3f-9ee0-c89c46c96082" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_155d576a-8a2e-4d9d-a524-2393876d380c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_464b350b-2447-47d8-a25c-ad5572aa6fd3" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_155d576a-8a2e-4d9d-a524-2393876d380c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_0985fd23-b8a1-40b2-bda4-bda806f10e24" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_464b350b-2447-47d8-a25c-ad5572aa6fd3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_0985fd23-b8a1-40b2-bda4-bda806f10e24" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedComputerSoftwarePeriodIncreaseDecrease_ec702b14-f1c1-4e21-a87a-c91b25b90095" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CapitalizedComputerSoftwarePeriodIncreaseDecrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_464b350b-2447-47d8-a25c-ad5572aa6fd3" xlink:to="loc_us-gaap_CapitalizedComputerSoftwarePeriodIncreaseDecrease_ec702b14-f1c1-4e21-a87a-c91b25b90095" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestCostsCapitalized_de5be416-1ddd-4b83-869d-a14311d29358" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InterestCostsCapitalized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_464b350b-2447-47d8-a25c-ad5572aa6fd3" xlink:to="loc_us-gaap_InterestCostsCapitalized_de5be416-1ddd-4b83-869d-a14311d29358" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestCostsIncurred_50ec537a-1446-4c9e-9bbd-fcea2a3ca25a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InterestCostsIncurred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_464b350b-2447-47d8-a25c-ad5572aa6fd3" xlink:to="loc_us-gaap_InterestCostsIncurred_50ec537a-1446-4c9e-9bbd-fcea2a3ca25a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Depreciation_0566ad2d-ec46-4e10-8645-145405749452" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Depreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_464b350b-2447-47d8-a25c-ad5572aa6fd3" xlink:to="loc_us-gaap_Depreciation_0566ad2d-ec46-4e10-8645-145405749452" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_98874a54-03db-462c-887a-2ac67b12ec70" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_464b350b-2447-47d8-a25c-ad5572aa6fd3" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_98874a54-03db-462c-887a-2ac67b12ec70" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_08ae895f-0cad-4294-9ba8-57b953983918" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_98874a54-03db-462c-887a-2ac67b12ec70" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_08ae895f-0cad-4294-9ba8-57b953983918" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_08ae895f-0cad-4294-9ba8-57b953983918_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_08ae895f-0cad-4294-9ba8-57b953983918" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_08ae895f-0cad-4294-9ba8-57b953983918_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_cc298165-5eda-4c9c-9f67-86a044f106a9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_08ae895f-0cad-4294-9ba8-57b953983918" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_cc298165-5eda-4c9c-9f67-86a044f106a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MachineryAndEquipmentMember_e8d9d76f-51c3-44f0-8d5f-25edfc8e1159" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_MachineryAndEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_cc298165-5eda-4c9c-9f67-86a044f106a9" xlink:to="loc_us-gaap_MachineryAndEquipmentMember_e8d9d76f-51c3-44f0-8d5f-25edfc8e1159" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareDevelopmentMember_7f2f1d7b-9a17-4c46-a418-37126fa486c9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SoftwareDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_cc298165-5eda-4c9c-9f67-86a044f106a9" xlink:to="loc_us-gaap_SoftwareDevelopmentMember_7f2f1d7b-9a17-4c46-a418-37126fa486c9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ComputerEquipmentAndSoftwareMember_85a7c387-144c-4e16-a8cd-f0449c2ce35f" xlink:href="tdup-20211231.xsd#tdup_ComputerEquipmentAndSoftwareMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_cc298165-5eda-4c9c-9f67-86a044f106a9" xlink:to="loc_tdup_ComputerEquipmentAndSoftwareMember_85a7c387-144c-4e16-a8cd-f0449c2ce35f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_bb9dead6-7fba-4380-a60d-83ce9d5a77d5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_cc298165-5eda-4c9c-9f67-86a044f106a9" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_bb9dead6-7fba-4380-a60d-83ce9d5a77d5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_eefdb98a-1430-4efc-8213-c26bfede5d1b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_cc298165-5eda-4c9c-9f67-86a044f106a9" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_eefdb98a-1430-4efc-8213-c26bfede5d1b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConstructionInProgressMember_d92fef37-1fe0-43a2-a354-564ab0450ff9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConstructionInProgressMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_cc298165-5eda-4c9c-9f67-86a044f106a9" xlink:to="loc_us-gaap_ConstructionInProgressMember_d92fef37-1fe0-43a2-a354-564ab0450ff9" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/AcquisitionofRemixNarrativeDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#AcquisitionofRemixNarrativeDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/AcquisitionofRemixNarrativeDetails" xlink:type="extended" id="i3c1ddfad6d8940cc8ae8b0900d9a3624_AcquisitionofRemixNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_6f70b574-f8f7-47c7-88e7-3e06023face3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_2eb5550c-35e4-4f0e-a1f0-79a4380ffc3e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_6f70b574-f8f7-47c7-88e7-3e06023face3" xlink:to="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_2eb5550c-35e4-4f0e-a1f0-79a4380ffc3e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_63415d1b-68fc-4886-9c24-c735c9ad8cc9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_6f70b574-f8f7-47c7-88e7-3e06023face3" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_63415d1b-68fc-4886-9c24-c735c9ad8cc9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued_bb402f99-7f71-4385-915f-d3ab51d5b500" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_6f70b574-f8f7-47c7-88e7-3e06023face3" xlink:to="loc_us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued_bb402f99-7f71-4385-915f-d3ab51d5b500" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionEquityInterestIssuedOrIssuableValueAssigned_6057c66d-975e-49c0-8ba0-c589f78bca42" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionEquityInterestIssuedOrIssuableValueAssigned"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_6f70b574-f8f7-47c7-88e7-3e06023face3" xlink:to="loc_us-gaap_BusinessAcquisitionEquityInterestIssuedOrIssuableValueAssigned_6057c66d-975e-49c0-8ba0-c589f78bca42" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual_a72de44b-fcbf-4f64-8340-0dfdc3ba22ee" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_6f70b574-f8f7-47c7-88e7-3e06023face3" xlink:to="loc_us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual_a72de44b-fcbf-4f64-8340-0dfdc3ba22ee" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationProFormaInformationEarningsOrLossOfAcquireeSinceAcquisitionDateActual_d144da71-1855-4107-8ce5-53287d1d2294" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationProFormaInformationEarningsOrLossOfAcquireeSinceAcquisitionDateActual"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_6f70b574-f8f7-47c7-88e7-3e06023face3" xlink:to="loc_us-gaap_BusinessCombinationProFormaInformationEarningsOrLossOfAcquireeSinceAcquisitionDateActual_d144da71-1855-4107-8ce5-53287d1d2294" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_5a383819-56a2-4c73-969f-bca3eea8a763" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_6f70b574-f8f7-47c7-88e7-3e06023face3" xlink:to="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_5a383819-56a2-4c73-969f-bca3eea8a763" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_088c7953-d946-4205-8151-b557348a8717" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_6f70b574-f8f7-47c7-88e7-3e06023face3" xlink:to="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_088c7953-d946-4205-8151-b557348a8717" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_4c171de7-5c5e-4c72-be66-a8f16054c785" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_6f70b574-f8f7-47c7-88e7-3e06023face3" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_4c171de7-5c5e-4c72-be66-a8f16054c785" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_153e09ee-2b8f-4038-b9fe-370fde1323eb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_4c171de7-5c5e-4c72-be66-a8f16054c785" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_153e09ee-2b8f-4038-b9fe-370fde1323eb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_153e09ee-2b8f-4038-b9fe-370fde1323eb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_153e09ee-2b8f-4038-b9fe-370fde1323eb" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_153e09ee-2b8f-4038-b9fe-370fde1323eb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_25b806e7-c38d-41af-ad59-e639042d77f3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_153e09ee-2b8f-4038-b9fe-370fde1323eb" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_25b806e7-c38d-41af-ad59-e639042d77f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_RemixGlobalADMember_b577b358-5a91-49bc-9e62-a29d0780c811" xlink:href="tdup-20211231.xsd#tdup_RemixGlobalADMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_25b806e7-c38d-41af-ad59-e639042d77f3" xlink:to="loc_tdup_RemixGlobalADMember_b577b358-5a91-49bc-9e62-a29d0780c811" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/AcquisitionofRemixEstimatedFairValuesoftheAssetsAcquiredandLiabilitiesDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#AcquisitionofRemixEstimatedFairValuesoftheAssetsAcquiredandLiabilitiesDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/AcquisitionofRemixEstimatedFairValuesoftheAssetsAcquiredandLiabilitiesDetails" xlink:type="extended" id="if711ddbd00c34664aa0710339c8a2c76_AcquisitionofRemixEstimatedFairValuesoftheAssetsAcquiredandLiabilitiesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_7d3612b1-cd77-4e2d-90ba-fba164c44c70" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_4815561b-f49b-47ef-9c54-4a03522af1e2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferredAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7d3612b1-cd77-4e2d-90ba-fba164c44c70" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_4815561b-f49b-47ef-9c54-4a03522af1e2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_33eb5f21-0475-4024-87b7-7548e4f6ec24" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_4815561b-f49b-47ef-9c54-4a03522af1e2" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_33eb5f21-0475-4024-87b7-7548e4f6ec24" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_2370e00c-af1b-404d-bbc8-4d533f8645e3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_4815561b-f49b-47ef-9c54-4a03522af1e2" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_2370e00c-af1b-404d-bbc8-4d533f8645e3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_e8d4b489-2ec6-45b3-92bd-c4540b64f857" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_4815561b-f49b-47ef-9c54-4a03522af1e2" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_e8d4b489-2ec6-45b3-92bd-c4540b64f857" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_1e20747d-ce3d-482e-9e1e-1eaa82ca8092" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7d3612b1-cd77-4e2d-90ba-fba164c44c70" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_1e20747d-ce3d-482e-9e1e-1eaa82ca8092" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_c6d7d2ac-e73d-4935-8caa-c7baef464bac" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_1e20747d-ce3d-482e-9e1e-1eaa82ca8092" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_c6d7d2ac-e73d-4935-8caa-c7baef464bac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_7442c674-37eb-48e6-85e8-f9147653e230" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_1e20747d-ce3d-482e-9e1e-1eaa82ca8092" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_7442c674-37eb-48e6-85e8-f9147653e230" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory_615951f3-db9e-4f27-ba27-753936a655b2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_1e20747d-ce3d-482e-9e1e-1eaa82ca8092" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory_615951f3-db9e-4f27-ba27-753936a655b2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_785f8cb6-e458-4c44-b1a7-0c1899eaa258" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_1e20747d-ce3d-482e-9e1e-1eaa82ca8092" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_785f8cb6-e458-4c44-b1a7-0c1899eaa258" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets_bcacaa39-ff62-429b-9710-1097d8203214" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_1e20747d-ce3d-482e-9e1e-1eaa82ca8092" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets_bcacaa39-ff62-429b-9710-1097d8203214" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_bfca28bd-3207-432a-b972-9c72baaa81c5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_1e20747d-ce3d-482e-9e1e-1eaa82ca8092" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_bfca28bd-3207-432a-b972-9c72baaa81c5" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_bb5dadca-86bc-4dd8-8118-dfe874b1e6ca" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_1e20747d-ce3d-482e-9e1e-1eaa82ca8092" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_bb5dadca-86bc-4dd8-8118-dfe874b1e6ca" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_9f1249f4-f2be-45d3-924f-4bee0000b75d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_1e20747d-ce3d-482e-9e1e-1eaa82ca8092" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_9f1249f4-f2be-45d3-924f-4bee0000b75d" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillRollForward_943d5e10-dc79-443e-953d-1f8e5307b349" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_1e20747d-ce3d-482e-9e1e-1eaa82ca8092" xlink:to="loc_us-gaap_GoodwillRollForward_943d5e10-dc79-443e-953d-1f8e5307b349" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_c9dbfa54-b69f-464f-b4a9-dbddf599e89a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_943d5e10-dc79-443e-953d-1f8e5307b349" xlink:to="loc_us-gaap_Goodwill_c9dbfa54-b69f-464f-b4a9-dbddf599e89a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillForeignCurrencyTranslationGainLoss_0c7f067c-81a6-4b0d-a017-fa30c54bb039" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillForeignCurrencyTranslationGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_943d5e10-dc79-443e-953d-1f8e5307b349" xlink:to="loc_us-gaap_GoodwillForeignCurrencyTranslationGainLoss_0c7f067c-81a6-4b0d-a017-fa30c54bb039" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_43414b55-c2dd-4ae0-b871-a7d029eac9e7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_0f5edfa2-fb33-4e62-8ebc-c509a98f35a3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_7d3612b1-cd77-4e2d-90ba-fba164c44c70" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_0f5edfa2-fb33-4e62-8ebc-c509a98f35a3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_7a0c99e5-1ac0-4c5d-b6d5-43a4c3b71fa7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_0f5edfa2-fb33-4e62-8ebc-c509a98f35a3" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_7a0c99e5-1ac0-4c5d-b6d5-43a4c3b71fa7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_7a0c99e5-1ac0-4c5d-b6d5-43a4c3b71fa7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_7a0c99e5-1ac0-4c5d-b6d5-43a4c3b71fa7" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_7a0c99e5-1ac0-4c5d-b6d5-43a4c3b71fa7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_6497443e-88f9-43ad-a1ea-9bb209515b7a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_7a0c99e5-1ac0-4c5d-b6d5-43a4c3b71fa7" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_6497443e-88f9-43ad-a1ea-9bb209515b7a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_RemixGlobalADMember_8cc6bfff-01de-4882-a80f-6a282dc365f0" xlink:href="tdup-20211231.xsd#tdup_RemixGlobalADMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_6497443e-88f9-43ad-a1ea-9bb209515b7a" xlink:to="loc_tdup_RemixGlobalADMember_8cc6bfff-01de-4882-a80f-6a282dc365f0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_2e3b5397-b2c7-434c-a1bb-00b40a8db1b1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_0f5edfa2-fb33-4e62-8ebc-c509a98f35a3" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_2e3b5397-b2c7-434c-a1bb-00b40a8db1b1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2e3b5397-b2c7-434c-a1bb-00b40a8db1b1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_2e3b5397-b2c7-434c-a1bb-00b40a8db1b1" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2e3b5397-b2c7-434c-a1bb-00b40a8db1b1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d8d0658a-f233-4f8e-a638-2577820891a5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_2e3b5397-b2c7-434c-a1bb-00b40a8db1b1" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d8d0658a-f233-4f8e-a638-2577820891a5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TrademarksMember_5cc65972-e374-4e5d-967e-d28ad46f12e8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TrademarksMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d8d0658a-f233-4f8e-a638-2577820891a5" xlink:to="loc_us-gaap_TrademarksMember_5cc65972-e374-4e5d-967e-d28ad46f12e8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_648a59a0-c974-464a-b39c-12727d333372" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DevelopedTechnologyRightsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d8d0658a-f233-4f8e-a638-2577820891a5" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_648a59a0-c974-464a-b39c-12727d333372" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_b4bb79d9-d7b8-4874-9404-8864454a642a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_d8d0658a-f233-4f8e-a638-2577820891a5" xlink:to="loc_us-gaap_CustomerRelationshipsMember_b4bb79d9-d7b8-4874-9404-8864454a642a" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/AcquisitionofRemixIntangibleAssetsDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#AcquisitionofRemixIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/AcquisitionofRemixIntangibleAssetsDetails" xlink:type="extended" id="id14e1419760c42a0a861c79df0a5d431_AcquisitionofRemixIntangibleAssetsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_27d816b6-5f5d-41c4-9915-909eecdf6e76" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_fe25a0d0-24d6-4228-8684-2e226b40561d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_27d816b6-5f5d-41c4-9915-909eecdf6e76" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_fe25a0d0-24d6-4228-8684-2e226b40561d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsTranslationAndPurchaseAccountingAdjustments_a74cbd98-6ef1-4044-9565-c83207c94388" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsTranslationAndPurchaseAccountingAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_27d816b6-5f5d-41c4-9915-909eecdf6e76" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsTranslationAndPurchaseAccountingAdjustments_a74cbd98-6ef1-4044-9565-c83207c94388" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_2df00172-99a6-49c7-b283-38b526724735" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_27d816b6-5f5d-41c4-9915-909eecdf6e76" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_2df00172-99a6-49c7-b283-38b526724735" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_56f0829e-4007-45dd-ad24-540ef3890363" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_27d816b6-5f5d-41c4-9915-909eecdf6e76" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_56f0829e-4007-45dd-ad24-540ef3890363" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_5e2995d5-7967-46f1-8dd6-80c3f417ac36" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_56f0829e-4007-45dd-ad24-540ef3890363" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_5e2995d5-7967-46f1-8dd6-80c3f417ac36" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5e2995d5-7967-46f1-8dd6-80c3f417ac36_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_5e2995d5-7967-46f1-8dd6-80c3f417ac36" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5e2995d5-7967-46f1-8dd6-80c3f417ac36_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_b54e4b85-bbdd-468c-83a7-8554277c4e20" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_5e2995d5-7967-46f1-8dd6-80c3f417ac36" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_b54e4b85-bbdd-468c-83a7-8554277c4e20" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_RemixGlobalADMember_00703a33-ebfd-4bba-8fbe-a80f062c909d" xlink:href="tdup-20211231.xsd#tdup_RemixGlobalADMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_b54e4b85-bbdd-468c-83a7-8554277c4e20" xlink:to="loc_tdup_RemixGlobalADMember_00703a33-ebfd-4bba-8fbe-a80f062c909d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_a2f61b01-e138-478f-bd67-16e29330eb88" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_56f0829e-4007-45dd-ad24-540ef3890363" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_a2f61b01-e138-478f-bd67-16e29330eb88" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_a2f61b01-e138-478f-bd67-16e29330eb88_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_a2f61b01-e138-478f-bd67-16e29330eb88" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_a2f61b01-e138-478f-bd67-16e29330eb88_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_6985d2ca-b2d3-4ca8-9c31-a131ab6e449c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_a2f61b01-e138-478f-bd67-16e29330eb88" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_6985d2ca-b2d3-4ca8-9c31-a131ab6e449c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_1f82b1f4-7bf5-4d80-aa87-dadfe80caccc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_6985d2ca-b2d3-4ca8-9c31-a131ab6e449c" xlink:to="loc_us-gaap_CustomerRelationshipsMember_1f82b1f4-7bf5-4d80-aa87-dadfe80caccc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_083e28b4-a547-4914-884e-820cc196755e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DevelopedTechnologyRightsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_6985d2ca-b2d3-4ca8-9c31-a131ab6e449c" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_083e28b4-a547-4914-884e-820cc196755e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_16ff6386-4b46-4296-af3c-e2f9e5ba04d4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_6985d2ca-b2d3-4ca8-9c31-a131ab6e449c" xlink:to="loc_us-gaap_TradeNamesMember_16ff6386-4b46-4296-af3c-e2f9e5ba04d4" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/GoodwillandOtherIntangibleAssetsScheduleofIntangibleAssetsDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#GoodwillandOtherIntangibleAssetsScheduleofIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/GoodwillandOtherIntangibleAssetsScheduleofIntangibleAssetsDetails" xlink:type="extended" id="i55157509b6a6445285c79c1ad3273265_GoodwillandOtherIntangibleAssetsScheduleofIntangibleAssetsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_94d62a38-cdc6-44bf-ab86-31a1159b8a6b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_def55651-0656-4761-b7a0-9b3c4d686af6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_94d62a38-cdc6-44bf-ab86-31a1159b8a6b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_def55651-0656-4761-b7a0-9b3c4d686af6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_580bd2aa-47ec-490d-ad6b-f42cc2fb25a8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_94d62a38-cdc6-44bf-ab86-31a1159b8a6b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_580bd2aa-47ec-490d-ad6b-f42cc2fb25a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_9165e621-6e3b-45ee-80d3-39f99a559b45" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_94d62a38-cdc6-44bf-ab86-31a1159b8a6b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_9165e621-6e3b-45ee-80d3-39f99a559b45" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_64ca6f3f-bbcb-4c47-bb91-f31d65d9254d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_94d62a38-cdc6-44bf-ab86-31a1159b8a6b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_64ca6f3f-bbcb-4c47-bb91-f31d65d9254d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_11c6f1c7-efed-46fe-bc6f-f977d981d8b1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_94d62a38-cdc6-44bf-ab86-31a1159b8a6b" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_11c6f1c7-efed-46fe-bc6f-f977d981d8b1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_8b75a4b8-79c7-4bbf-8438-f43345a9f952" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_11c6f1c7-efed-46fe-bc6f-f977d981d8b1" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_8b75a4b8-79c7-4bbf-8438-f43345a9f952" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_8b75a4b8-79c7-4bbf-8438-f43345a9f952_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_8b75a4b8-79c7-4bbf-8438-f43345a9f952" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_8b75a4b8-79c7-4bbf-8438-f43345a9f952_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_5b4c44ed-367d-42cc-ac2f-6a62e8e60a23" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_8b75a4b8-79c7-4bbf-8438-f43345a9f952" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_5b4c44ed-367d-42cc-ac2f-6a62e8e60a23" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_83af3b95-479c-47ed-a075-26109cc16fe5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_5b4c44ed-367d-42cc-ac2f-6a62e8e60a23" xlink:to="loc_us-gaap_CustomerRelationshipsMember_83af3b95-479c-47ed-a075-26109cc16fe5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_a1f383f8-31ef-4354-beda-540bbb531bfc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DevelopedTechnologyRightsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_5b4c44ed-367d-42cc-ac2f-6a62e8e60a23" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_a1f383f8-31ef-4354-beda-540bbb531bfc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TrademarksMember_640cfe1a-d31d-4a4d-9284-b28fbdca33b3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TrademarksMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_5b4c44ed-367d-42cc-ac2f-6a62e8e60a23" xlink:to="loc_us-gaap_TrademarksMember_640cfe1a-d31d-4a4d-9284-b28fbdca33b3" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/LeaseAgreementsNarrativeDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#LeaseAgreementsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/LeaseAgreementsNarrativeDetails" xlink:type="extended" id="ifdfaebc16025425b963275a1ddb83f9f_LeaseAgreementsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_d1842965-29b4-4f30-98c3-2d5bd3140278" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseTermOfContract_f23a992a-67c5-4f52-9db1-68b7562ba53f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeOperatingLeaseTermOfContract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_d1842965-29b4-4f30-98c3-2d5bd3140278" xlink:to="loc_us-gaap_LesseeOperatingLeaseTermOfContract_f23a992a-67c5-4f52-9db1-68b7562ba53f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_74603fb5-7e46-4972-aa1b-c5e6fd3757bb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeOperatingLeaseRenewalTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_d1842965-29b4-4f30-98c3-2d5bd3140278" xlink:to="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_74603fb5-7e46-4972-aa1b-c5e6fd3757bb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1_25d627b3-88e4-4d37-91ff-28db957e079d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_d1842965-29b4-4f30-98c3-2d5bd3140278" xlink:to="loc_us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1_25d627b3-88e4-4d37-91ff-28db957e079d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_LesseeOperatingLeaseLeaseNotYetCommencedLiability_c0353870-a56e-42ba-865a-d4eb181025cc" xlink:href="tdup-20211231.xsd#tdup_LesseeOperatingLeaseLeaseNotYetCommencedLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_d1842965-29b4-4f30-98c3-2d5bd3140278" xlink:to="loc_tdup_LesseeOperatingLeaseLeaseNotYetCommencedLiability_c0353870-a56e-42ba-865a-d4eb181025cc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecurityDeposit_598d1440-4e06-4a35-95a4-8c4ce6eff30a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SecurityDeposit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_d1842965-29b4-4f30-98c3-2d5bd3140278" xlink:to="loc_us-gaap_SecurityDeposit_598d1440-4e06-4a35-95a4-8c4ce6eff30a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_52e4bf70-90f3-447a-bfb3-313dc1604ea7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_d1842965-29b4-4f30-98c3-2d5bd3140278" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_52e4bf70-90f3-447a-bfb3-313dc1604ea7" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseCost_c7953a0e-3a38-4d20-9e0d-b52875b73c71" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_d1842965-29b4-4f30-98c3-2d5bd3140278" xlink:to="loc_us-gaap_LeaseCost_c7953a0e-3a38-4d20-9e0d-b52875b73c71" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseAndRentalExpense_f9b3b93d-35b8-4ce7-8988-b939ed863890" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LeaseAndRentalExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_d1842965-29b4-4f30-98c3-2d5bd3140278" xlink:to="loc_us-gaap_LeaseAndRentalExpense_f9b3b93d-35b8-4ce7-8988-b939ed863890" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_9ae37409-5773-45f9-8dcf-2a6cd7b643ff" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_d1842965-29b4-4f30-98c3-2d5bd3140278" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_9ae37409-5773-45f9-8dcf-2a6cd7b643ff" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_c2246fc2-a002-4394-9d07-a2697813b870" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_9ae37409-5773-45f9-8dcf-2a6cd7b643ff" xlink:to="loc_srt_RangeAxis_c2246fc2-a002-4394-9d07-a2697813b870" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_c2246fc2-a002-4394-9d07-a2697813b870_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_c2246fc2-a002-4394-9d07-a2697813b870" xlink:to="loc_srt_RangeMember_c2246fc2-a002-4394-9d07-a2697813b870_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_da0d97a0-36ea-4037-9e5c-d1ce046a6465" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_c2246fc2-a002-4394-9d07-a2697813b870" xlink:to="loc_srt_RangeMember_da0d97a0-36ea-4037-9e5c-d1ce046a6465" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_e4995dac-eee2-456e-b2d8-3955e537daab" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_da0d97a0-36ea-4037-9e5c-d1ce046a6465" xlink:to="loc_srt_MinimumMember_e4995dac-eee2-456e-b2d8-3955e537daab" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_47cd5564-1b28-49d4-8838-12bd73fc59d3" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_da0d97a0-36ea-4037-9e5c-d1ce046a6465" xlink:to="loc_srt_MaximumMember_47cd5564-1b28-49d4-8838-12bd73fc59d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_935336a5-ed4f-46e5-aa80-4d2f9ca57ee2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_9ae37409-5773-45f9-8dcf-2a6cd7b643ff" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_935336a5-ed4f-46e5-aa80-4d2f9ca57ee2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_935336a5-ed4f-46e5-aa80-4d2f9ca57ee2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_935336a5-ed4f-46e5-aa80-4d2f9ca57ee2" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_935336a5-ed4f-46e5-aa80-4d2f9ca57ee2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_7ac31ad0-2a0b-42ea-9343-044e399ed1c2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_935336a5-ed4f-46e5-aa80-4d2f9ca57ee2" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_7ac31ad0-2a0b-42ea-9343-044e399ed1c2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BuildingMember_069f350e-3295-4f25-afae-0ec335ee3fdc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BuildingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_7ac31ad0-2a0b-42ea-9343-044e399ed1c2" xlink:to="loc_us-gaap_BuildingMember_069f350e-3295-4f25-afae-0ec335ee3fdc" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/LongtermDebtandConvertiblePreferredStockWarrantsNarrativeDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#LongtermDebtandConvertiblePreferredStockWarrantsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/LongtermDebtandConvertiblePreferredStockWarrantsNarrativeDetails" xlink:type="extended" id="ic9813850139646149cf081daf99263cd_LongtermDebtandConvertiblePreferredStockWarrantsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_75f40a81-0313-476a-9662-ac489ca5a806" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_DebtInstrumentMaximumBorrowingCapacity_6371247a-4a23-46ec-a956-656c109f526e" xlink:href="tdup-20211231.xsd#tdup_DebtInstrumentMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_75f40a81-0313-476a-9662-ac489ca5a806" xlink:to="loc_tdup_DebtInstrumentMaximumBorrowingCapacity_6371247a-4a23-46ec-a956-656c109f526e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_DebtInstrumentNumberOfAmendments_538d2fb0-e822-47e0-b9dd-d5bf0ff56630" xlink:href="tdup-20211231.xsd#tdup_DebtInstrumentNumberOfAmendments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_75f40a81-0313-476a-9662-ac489ca5a806" xlink:to="loc_tdup_DebtInstrumentNumberOfAmendments_538d2fb0-e822-47e0-b9dd-d5bf0ff56630" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebt_149135d6-7785-4874-88f3-ef038f016764" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProceedsFromIssuanceOfLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_75f40a81-0313-476a-9662-ac489ca5a806" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebt_149135d6-7785-4874-88f3-ef038f016764" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_d7994f01-0624-43da-b041-74f9072b4785" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_75f40a81-0313-476a-9662-ac489ca5a806" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_d7994f01-0624-43da-b041-74f9072b4785" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_321f55a0-3cea-44f3-8c36-e560045b91f9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_75f40a81-0313-476a-9662-ac489ca5a806" xlink:to="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_321f55a0-3cea-44f3-8c36-e560045b91f9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_642b41e7-f6bd-4b2c-90ab-5c879ab057cb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_75f40a81-0313-476a-9662-ac489ca5a806" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_642b41e7-f6bd-4b2c-90ab-5c879ab057cb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_1999da68-f60c-4276-b282-30ac7e489429" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_75f40a81-0313-476a-9662-ac489ca5a806" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_1999da68-f60c-4276-b282-30ac7e489429" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_DebtInstrumentFloorPercentage_5ad4ece9-4b17-4b0c-8105-d9a66f041a0f" xlink:href="tdup-20211231.xsd#tdup_DebtInstrumentFloorPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_75f40a81-0313-476a-9662-ac489ca5a806" xlink:to="loc_tdup_DebtInstrumentFloorPercentage_5ad4ece9-4b17-4b0c-8105-d9a66f041a0f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_910ecabc-9555-4f0a-9cf6-8e05c4be3531" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_75f40a81-0313-476a-9662-ac489ca5a806" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_910ecabc-9555-4f0a-9cf6-8e05c4be3531" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstanding_03228f98-2d08-4b09-9867-e283f7d0ba50" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_WarrantsAndRightsOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_75f40a81-0313-476a-9662-ac489ca5a806" xlink:to="loc_us-gaap_WarrantsAndRightsOutstanding_03228f98-2d08-4b09-9867-e283f7d0ba50" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_981b54f0-e765-48b5-a6f4-67081955ae21" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfWarrantOrRightOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_75f40a81-0313-476a-9662-ac489ca5a806" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_981b54f0-e765-48b5-a6f4-67081955ae21" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_fb7bcaf0-80f3-481e-b82f-c6a33e74a627" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_75f40a81-0313-476a-9662-ac489ca5a806" xlink:to="loc_us-gaap_DebtInstrumentTable_fb7bcaf0-80f3-481e-b82f-c6a33e74a627" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_77ebf89f-7a1d-404f-be56-1bb84dd93162" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_fb7bcaf0-80f3-481e-b82f-c6a33e74a627" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_77ebf89f-7a1d-404f-be56-1bb84dd93162" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_77ebf89f-7a1d-404f-be56-1bb84dd93162_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_77ebf89f-7a1d-404f-be56-1bb84dd93162" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_77ebf89f-7a1d-404f-be56-1bb84dd93162_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_ca4f019c-3bc9-495b-8cb5-4ffd49446e01" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_77ebf89f-7a1d-404f-be56-1bb84dd93162" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_ca4f019c-3bc9-495b-8cb5-4ffd49446e01" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MediumTermNotesMember_6f67cba1-26f3-497d-8989-05e5b9ab9f2e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_MediumTermNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_ca4f019c-3bc9-495b-8cb5-4ffd49446e01" xlink:to="loc_us-gaap_MediumTermNotesMember_6f67cba1-26f3-497d-8989-05e5b9ab9f2e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_9a6671a4-7283-4ff1-ad8c-3ca47d4eb3bf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_fb7bcaf0-80f3-481e-b82f-c6a33e74a627" xlink:to="loc_us-gaap_DebtInstrumentAxis_9a6671a4-7283-4ff1-ad8c-3ca47d4eb3bf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_9a6671a4-7283-4ff1-ad8c-3ca47d4eb3bf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_9a6671a4-7283-4ff1-ad8c-3ca47d4eb3bf" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_9a6671a4-7283-4ff1-ad8c-3ca47d4eb3bf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_8066e941-b1a8-49e3-bc80-346ee0004b5a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_9a6671a4-7283-4ff1-ad8c-3ca47d4eb3bf" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_8066e941-b1a8-49e3-bc80-346ee0004b5a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_WesternAllianceBankTermLoanAndSecurityAgreementMember_eab39108-0590-4274-b981-b0bcf9240bb5" xlink:href="tdup-20211231.xsd#tdup_WesternAllianceBankTermLoanAndSecurityAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_8066e941-b1a8-49e3-bc80-346ee0004b5a" xlink:to="loc_tdup_WesternAllianceBankTermLoanAndSecurityAgreementMember_eab39108-0590-4274-b981-b0bcf9240bb5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_d76ba939-37d2-4284-a2bd-bf1dcbffca7a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_fb7bcaf0-80f3-481e-b82f-c6a33e74a627" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_d76ba939-37d2-4284-a2bd-bf1dcbffca7a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_d76ba939-37d2-4284-a2bd-bf1dcbffca7a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_d76ba939-37d2-4284-a2bd-bf1dcbffca7a" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_d76ba939-37d2-4284-a2bd-bf1dcbffca7a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_962a3392-4fdf-4ca9-bc50-e378dd74cfe9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_d76ba939-37d2-4284-a2bd-bf1dcbffca7a" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_962a3392-4fdf-4ca9-bc50-e378dd74cfe9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SeriesE1ConvertiblePreferredStockMember_b57f16dd-44b6-4623-b2a4-9d48aee8d78f" xlink:href="tdup-20211231.xsd#tdup_SeriesE1ConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_962a3392-4fdf-4ca9-bc50-e378dd74cfe9" xlink:to="loc_tdup_SeriesE1ConvertiblePreferredStockMember_b57f16dd-44b6-4623-b2a4-9d48aee8d78f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ClassBCommonStockWarrantMember_3db4baea-9f1c-4f80-bd44-409146fc18e9" xlink:href="tdup-20211231.xsd#tdup_ClassBCommonStockWarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_962a3392-4fdf-4ca9-bc50-e378dd74cfe9" xlink:to="loc_tdup_ClassBCommonStockWarrantMember_3db4baea-9f1c-4f80-bd44-409146fc18e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_13e3a53d-775a-4a97-872d-7a00dfbd860c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_fb7bcaf0-80f3-481e-b82f-c6a33e74a627" xlink:to="loc_us-gaap_VariableRateAxis_13e3a53d-775a-4a97-872d-7a00dfbd860c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_13e3a53d-775a-4a97-872d-7a00dfbd860c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_13e3a53d-775a-4a97-872d-7a00dfbd860c" xlink:to="loc_us-gaap_VariableRateDomain_13e3a53d-775a-4a97-872d-7a00dfbd860c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_24e16fe5-daea-410f-a046-e93536022fc0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_13e3a53d-775a-4a97-872d-7a00dfbd860c" xlink:to="loc_us-gaap_VariableRateDomain_24e16fe5-daea-410f-a046-e93536022fc0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrimeRateMember_0d28f9f6-1983-4800-b802-a80c1ddf8e57" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PrimeRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_24e16fe5-daea-410f-a046-e93536022fc0" xlink:to="loc_us-gaap_PrimeRateMember_0d28f9f6-1983-4800-b802-a80c1ddf8e57" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_b94b688a-cbcb-4736-918e-4f7915b4b9bb" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_fb7bcaf0-80f3-481e-b82f-c6a33e74a627" xlink:to="loc_srt_RangeAxis_b94b688a-cbcb-4736-918e-4f7915b4b9bb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_b94b688a-cbcb-4736-918e-4f7915b4b9bb_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_b94b688a-cbcb-4736-918e-4f7915b4b9bb" xlink:to="loc_srt_RangeMember_b94b688a-cbcb-4736-918e-4f7915b4b9bb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_50a684a1-1b59-4f45-8fcd-e90b8ac8e504" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_b94b688a-cbcb-4736-918e-4f7915b4b9bb" xlink:to="loc_srt_RangeMember_50a684a1-1b59-4f45-8fcd-e90b8ac8e504" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_31aa2804-64f1-4391-b21d-ab02bd6a80d2" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_50a684a1-1b59-4f45-8fcd-e90b8ac8e504" xlink:to="loc_srt_MinimumMember_31aa2804-64f1-4391-b21d-ab02bd6a80d2" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/LongtermDebtandConvertiblePreferredStockWarrantsScheduleofWarrantsIssuedDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#LongtermDebtandConvertiblePreferredStockWarrantsScheduleofWarrantsIssuedDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/LongtermDebtandConvertiblePreferredStockWarrantsScheduleofWarrantsIssuedDetails" xlink:type="extended" id="i40aee6038f424f5da52d3e9843f47f6a_LongtermDebtandConvertiblePreferredStockWarrantsScheduleofWarrantsIssuedDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_ccf2efce-a9d9-4cda-89f1-0806f858b924" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_d436d604-16c1-486b-8619-d7698c7dfdcc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccf2efce-a9d9-4cda-89f1-0806f858b924" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_d436d604-16c1-486b-8619-d7698c7dfdcc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_f598c2f7-9567-430e-8c27-a49773195e94" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfWarrantOrRightOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccf2efce-a9d9-4cda-89f1-0806f858b924" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_f598c2f7-9567-430e-8c27-a49773195e94" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_fdc2fcca-c852-407a-8fe5-448d2a07f8d6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ccf2efce-a9d9-4cda-89f1-0806f858b924" xlink:to="loc_us-gaap_DebtInstrumentTable_fdc2fcca-c852-407a-8fe5-448d2a07f8d6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_211884aa-2d71-4260-b81c-9b17e501d71a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_fdc2fcca-c852-407a-8fe5-448d2a07f8d6" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_211884aa-2d71-4260-b81c-9b17e501d71a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_211884aa-2d71-4260-b81c-9b17e501d71a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_211884aa-2d71-4260-b81c-9b17e501d71a" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_211884aa-2d71-4260-b81c-9b17e501d71a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_abc5d755-978a-4de9-809f-fe2ca9a1893d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_211884aa-2d71-4260-b81c-9b17e501d71a" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_abc5d755-978a-4de9-809f-fe2ca9a1893d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SeriesDConvertiblePreferredStockIssued1222015Member_383a4a88-435e-4707-9f61-2911ef67182d" xlink:href="tdup-20211231.xsd#tdup_SeriesDConvertiblePreferredStockIssued1222015Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_abc5d755-978a-4de9-809f-fe2ca9a1893d" xlink:to="loc_tdup_SeriesDConvertiblePreferredStockIssued1222015Member_383a4a88-435e-4707-9f61-2911ef67182d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SeriesDConvertiblePreferredStockIssued4202015Member_5ba0396f-1e49-4aac-8b88-9c0d52c44c51" xlink:href="tdup-20211231.xsd#tdup_SeriesDConvertiblePreferredStockIssued4202015Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_abc5d755-978a-4de9-809f-fe2ca9a1893d" xlink:to="loc_tdup_SeriesDConvertiblePreferredStockIssued4202015Member_5ba0396f-1e49-4aac-8b88-9c0d52c44c51" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SeriesE1ConvertiblePreferredStockIssued272019Member_0bd00025-f6dd-495c-827b-57118b373ad8" xlink:href="tdup-20211231.xsd#tdup_SeriesE1ConvertiblePreferredStockIssued272019Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_abc5d755-978a-4de9-809f-fe2ca9a1893d" xlink:to="loc_tdup_SeriesE1ConvertiblePreferredStockIssued272019Member_0bd00025-f6dd-495c-827b-57118b373ad8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SeriesFConvertiblePreferredStockIssued5292020Member_40ef8ed9-e9bd-4b70-b592-2c32c6e15725" xlink:href="tdup-20211231.xsd#tdup_SeriesFConvertiblePreferredStockIssued5292020Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_abc5d755-978a-4de9-809f-fe2ca9a1893d" xlink:to="loc_tdup_SeriesFConvertiblePreferredStockIssued5292020Member_40ef8ed9-e9bd-4b70-b592-2c32c6e15725" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SeriesE1ConvertiblePreferredStockIssued8142020Member_8350662f-90e0-4bab-825f-b17771ee3d2a" xlink:href="tdup-20211231.xsd#tdup_SeriesE1ConvertiblePreferredStockIssued8142020Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_abc5d755-978a-4de9-809f-fe2ca9a1893d" xlink:to="loc_tdup_SeriesE1ConvertiblePreferredStockIssued8142020Member_8350662f-90e0-4bab-825f-b17771ee3d2a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SeriesE1ConvertiblePreferredStockIssued11252020Member_7cf23303-12c7-4d56-9893-6567941739a4" xlink:href="tdup-20211231.xsd#tdup_SeriesE1ConvertiblePreferredStockIssued11252020Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_abc5d755-978a-4de9-809f-fe2ca9a1893d" xlink:to="loc_tdup_SeriesE1ConvertiblePreferredStockIssued11252020Member_7cf23303-12c7-4d56-9893-6567941739a4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SeriesE1ConvertiblePreferredStockIssued2282021Member_6ccfecff-656b-4cae-a1ae-ef8bf3006542" xlink:href="tdup-20211231.xsd#tdup_SeriesE1ConvertiblePreferredStockIssued2282021Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_abc5d755-978a-4de9-809f-fe2ca9a1893d" xlink:to="loc_tdup_SeriesE1ConvertiblePreferredStockIssued2282021Member_6ccfecff-656b-4cae-a1ae-ef8bf3006542" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/CommonStockandCommonStockWarrantsNarrativeDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#CommonStockandCommonStockWarrantsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/CommonStockandCommonStockWarrantsNarrativeDetails" xlink:type="extended" id="i401016a7a94043cbae6947fedafdbd7e_CommonStockandCommonStockWarrantsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_d249a38f-1e8d-4666-bffe-b3b950fb80b8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_NumberOfVotesForEachShareOfCommonStockHeld_2e63c284-f52d-4cf5-b77d-222ba2d9abf5" xlink:href="tdup-20211231.xsd#tdup_NumberOfVotesForEachShareOfCommonStockHeld"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_d249a38f-1e8d-4666-bffe-b3b950fb80b8" xlink:to="loc_tdup_NumberOfVotesForEachShareOfCommonStockHeld_2e63c284-f52d-4cf5-b77d-222ba2d9abf5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_StockholdersEquityConversionOfStockRatio_9cd2a8d3-498e-496d-a0ae-4aa01d852286" xlink:href="tdup-20211231.xsd#tdup_StockholdersEquityConversionOfStockRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_d249a38f-1e8d-4666-bffe-b3b950fb80b8" xlink:to="loc_tdup_StockholdersEquityConversionOfStockRatio_9cd2a8d3-498e-496d-a0ae-4aa01d852286" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_d73577df-8a3d-4189-bb6f-913d83cb764a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_d249a38f-1e8d-4666-bffe-b3b950fb80b8" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_d73577df-8a3d-4189-bb6f-913d83cb764a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_83c01381-d7c4-495a-a30c-2aebe288ed43" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfWarrantOrRightOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_d249a38f-1e8d-4666-bffe-b3b950fb80b8" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_83c01381-d7c4-495a-a30c-2aebe288ed43" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_889268b9-3c21-4788-9fcb-a3afe1f3f1ce" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_d249a38f-1e8d-4666-bffe-b3b950fb80b8" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_889268b9-3c21-4788-9fcb-a3afe1f3f1ce" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_d1eff3f2-fa96-41d8-ad1a-ffe93e9d0fb0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_d249a38f-1e8d-4666-bffe-b3b950fb80b8" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_d1eff3f2-fa96-41d8-ad1a-ffe93e9d0fb0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_6f96ace2-7beb-4f75-8429-1b176bad13b0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_d249a38f-1e8d-4666-bffe-b3b950fb80b8" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_6f96ace2-7beb-4f75-8429-1b176bad13b0" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfStockIssuanceCosts_0f3d79fe-e88c-461b-8b3c-7c48dfaa5a18" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsOfStockIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_d249a38f-1e8d-4666-bffe-b3b950fb80b8" xlink:to="loc_us-gaap_PaymentsOfStockIssuanceCosts_0f3d79fe-e88c-461b-8b3c-7c48dfaa5a18" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_StockIssuedDuringPeriodSharesWarrantsIssued_1b85d737-9354-4df1-b944-95a1247012e6" xlink:href="tdup-20211231.xsd#tdup_StockIssuedDuringPeriodSharesWarrantsIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_d249a38f-1e8d-4666-bffe-b3b950fb80b8" xlink:to="loc_tdup_StockIssuedDuringPeriodSharesWarrantsIssued_1b85d737-9354-4df1-b944-95a1247012e6" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_a44afaf5-2343-410a-a46f-7ab5c2e2f4d8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_d249a38f-1e8d-4666-bffe-b3b950fb80b8" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_a44afaf5-2343-410a-a46f-7ab5c2e2f4d8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_f1260201-5606-4fa7-9d8b-ca8db2bc12bd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_a44afaf5-2343-410a-a46f-7ab5c2e2f4d8" xlink:to="loc_us-gaap_StatementClassOfStockAxis_f1260201-5606-4fa7-9d8b-ca8db2bc12bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_f1260201-5606-4fa7-9d8b-ca8db2bc12bd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_f1260201-5606-4fa7-9d8b-ca8db2bc12bd" xlink:to="loc_us-gaap_ClassOfStockDomain_f1260201-5606-4fa7-9d8b-ca8db2bc12bd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_24e4553c-addf-4b88-bc6a-03f5d1d37e38" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_f1260201-5606-4fa7-9d8b-ca8db2bc12bd" xlink:to="loc_us-gaap_ClassOfStockDomain_24e4553c-addf-4b88-bc6a-03f5d1d37e38" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_216ffed6-2d01-4065-86fb-b015caeab7cd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_24e4553c-addf-4b88-bc6a-03f5d1d37e38" xlink:to="loc_us-gaap_CommonClassAMember_216ffed6-2d01-4065-86fb-b015caeab7cd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_16ee124d-e879-4604-b000-4ee44a7446d8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_24e4553c-addf-4b88-bc6a-03f5d1d37e38" xlink:to="loc_us-gaap_CommonClassBMember_16ee124d-e879-4604-b000-4ee44a7446d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_a77f5865-127d-48d3-9475-90cb29b062f1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_a44afaf5-2343-410a-a46f-7ab5c2e2f4d8" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_a77f5865-127d-48d3-9475-90cb29b062f1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_a77f5865-127d-48d3-9475-90cb29b062f1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_a77f5865-127d-48d3-9475-90cb29b062f1" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_a77f5865-127d-48d3-9475-90cb29b062f1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_cff49153-db56-4405-a9e8-73741839a27e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_a77f5865-127d-48d3-9475-90cb29b062f1" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_cff49153-db56-4405-a9e8-73741839a27e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ClassBCommonStockWarrantMember_86582d05-c8f2-4fbf-934f-74c4eb9ccbf0" xlink:href="tdup-20211231.xsd#tdup_ClassBCommonStockWarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_cff49153-db56-4405-a9e8-73741839a27e" xlink:to="loc_tdup_ClassBCommonStockWarrantMember_86582d05-c8f2-4fbf-934f-74c4eb9ccbf0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_223e4dcb-62d7-4583-8b83-dae8c39a6f78" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_a44afaf5-2343-410a-a46f-7ab5c2e2f4d8" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_223e4dcb-62d7-4583-8b83-dae8c39a6f78" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_223e4dcb-62d7-4583-8b83-dae8c39a6f78_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_223e4dcb-62d7-4583-8b83-dae8c39a6f78" xlink:to="loc_us-gaap_EquityComponentDomain_223e4dcb-62d7-4583-8b83-dae8c39a6f78_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_89a0e6d5-8ace-4bb3-bd5b-d37026d82609" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_223e4dcb-62d7-4583-8b83-dae8c39a6f78" xlink:to="loc_us-gaap_EquityComponentDomain_89a0e6d5-8ace-4bb3-bd5b-d37026d82609" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_06c02998-9404-4983-a447-f7b7f5ce2906" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_89a0e6d5-8ace-4bb3-bd5b-d37026d82609" xlink:to="loc_us-gaap_CommonStockMember_06c02998-9404-4983-a447-f7b7f5ce2906" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_a671314e-26a7-4516-992b-288d2d9b8a74" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_a44afaf5-2343-410a-a46f-7ab5c2e2f4d8" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_a671314e-26a7-4516-992b-288d2d9b8a74" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_a671314e-26a7-4516-992b-288d2d9b8a74_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_a671314e-26a7-4516-992b-288d2d9b8a74" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_a671314e-26a7-4516-992b-288d2d9b8a74_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_463f86ca-fb01-466b-a855-90f065487305" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_a671314e-26a7-4516-992b-288d2d9b8a74" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_463f86ca-fb01-466b-a855-90f065487305" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_FollowOnPublicOfferingMember_ac1dfc2c-b9e4-43ef-8f87-d3db60b28e68" xlink:href="tdup-20211231.xsd#tdup_FollowOnPublicOfferingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_463f86ca-fb01-466b-a855-90f065487305" xlink:to="loc_tdup_FollowOnPublicOfferingMember_ac1dfc2c-b9e4-43ef-8f87-d3db60b28e68" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OverAllotmentOptionMember_fce43839-c01f-4e2a-91bf-2c358371c8a5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OverAllotmentOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_463f86ca-fb01-466b-a855-90f065487305" xlink:to="loc_us-gaap_OverAllotmentOptionMember_fce43839-c01f-4e2a-91bf-2c358371c8a5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_3ed3822a-5430-4199-a877-400c60ca27f0" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_a44afaf5-2343-410a-a46f-7ab5c2e2f4d8" xlink:to="loc_dei_LegalEntityAxis_3ed3822a-5430-4199-a877-400c60ca27f0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_3ed3822a-5430-4199-a877-400c60ca27f0_default" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_3ed3822a-5430-4199-a877-400c60ca27f0" xlink:to="loc_dei_EntityDomain_3ed3822a-5430-4199-a877-400c60ca27f0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_60c88df7-f3b2-4662-beee-bdbd89421e52" xlink:href="https://xbrl.sec.gov/dei/2021q4/dei-2021q4.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_3ed3822a-5430-4199-a877-400c60ca27f0" xlink:to="loc_dei_EntityDomain_60c88df7-f3b2-4662-beee-bdbd89421e52" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SellingStockholdersMember_e07a7044-b207-4c38-9537-864b0219cbbf" xlink:href="tdup-20211231.xsd#tdup_SellingStockholdersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_60c88df7-f3b2-4662-beee-bdbd89421e52" xlink:to="loc_tdup_SellingStockholdersMember_e07a7044-b207-4c38-9537-864b0219cbbf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_71a683cc-99d2-4a5b-a3e2-bc2276d34447" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_a44afaf5-2343-410a-a46f-7ab5c2e2f4d8" xlink:to="loc_us-gaap_AwardTypeAxis_71a683cc-99d2-4a5b-a3e2-bc2276d34447" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_71a683cc-99d2-4a5b-a3e2-bc2276d34447_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_71a683cc-99d2-4a5b-a3e2-bc2276d34447" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_71a683cc-99d2-4a5b-a3e2-bc2276d34447_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_45dd5d86-aac3-496e-9fee-53bc88056456" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_71a683cc-99d2-4a5b-a3e2-bc2276d34447" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_45dd5d86-aac3-496e-9fee-53bc88056456" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_b1c8cb15-565d-4a42-89a1-6c07e9648ab9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_45dd5d86-aac3-496e-9fee-53bc88056456" xlink:to="loc_us-gaap_EmployeeStockOptionMember_b1c8cb15-565d-4a42-89a1-6c07e9648ab9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_00e6ce92-eb36-484d-aeb4-6fea6364994e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_a44afaf5-2343-410a-a46f-7ab5c2e2f4d8" xlink:to="loc_us-gaap_PlanNameAxis_00e6ce92-eb36-484d-aeb4-6fea6364994e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_00e6ce92-eb36-484d-aeb4-6fea6364994e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_00e6ce92-eb36-484d-aeb4-6fea6364994e" xlink:to="loc_us-gaap_PlanNameDomain_00e6ce92-eb36-484d-aeb4-6fea6364994e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_a3b88950-a444-4968-957a-ca2594a5c837" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_00e6ce92-eb36-484d-aeb4-6fea6364994e" xlink:to="loc_us-gaap_PlanNameDomain_a3b88950-a444-4968-957a-ca2594a5c837" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_StockIncentivePlan2010Member_abc53fcc-f3be-4945-9b03-513c6a073de5" xlink:href="tdup-20211231.xsd#tdup_StockIncentivePlan2010Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_a3b88950-a444-4968-957a-ca2594a5c837" xlink:to="loc_tdup_StockIncentivePlan2010Member_abc53fcc-f3be-4945-9b03-513c6a073de5" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/CommonStockandCommonStockWarrantsScheduleofCommonStockIssuedandOutstandingDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#CommonStockandCommonStockWarrantsScheduleofCommonStockIssuedandOutstandingDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/CommonStockandCommonStockWarrantsScheduleofCommonStockIssuedandOutstandingDetails" xlink:type="extended" id="i75c6a2b7db7f444ebde040799cc2736b_CommonStockandCommonStockWarrantsScheduleofCommonStockIssuedandOutstandingDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_6b08f703-7168-4c80-9964-e40f5e23e1d4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_884d0b91-f276-451a-a871-0bf06afbc5e6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_6b08f703-7168-4c80-9964-e40f5e23e1d4" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_884d0b91-f276-451a-a871-0bf06afbc5e6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_3e5793ed-6608-4369-b333-fb91f9633a3c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_6b08f703-7168-4c80-9964-e40f5e23e1d4" xlink:to="loc_us-gaap_CommonStockSharesIssued_3e5793ed-6608-4369-b333-fb91f9633a3c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_71795d99-9340-491b-8347-0debebccd96b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_6b08f703-7168-4c80-9964-e40f5e23e1d4" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_71795d99-9340-491b-8347-0debebccd96b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_5aee24ba-ca13-41e5-b7e6-ff8cc9ee63b5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_6b08f703-7168-4c80-9964-e40f5e23e1d4" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_5aee24ba-ca13-41e5-b7e6-ff8cc9ee63b5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_18825fa9-08be-45f5-a261-ba6a3ffde52d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_5aee24ba-ca13-41e5-b7e6-ff8cc9ee63b5" xlink:to="loc_us-gaap_StatementClassOfStockAxis_18825fa9-08be-45f5-a261-ba6a3ffde52d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_18825fa9-08be-45f5-a261-ba6a3ffde52d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_18825fa9-08be-45f5-a261-ba6a3ffde52d" xlink:to="loc_us-gaap_ClassOfStockDomain_18825fa9-08be-45f5-a261-ba6a3ffde52d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_14988e9f-f719-4135-84e0-d68f3155988c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_18825fa9-08be-45f5-a261-ba6a3ffde52d" xlink:to="loc_us-gaap_ClassOfStockDomain_14988e9f-f719-4135-84e0-d68f3155988c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_1eb5f181-18e6-4f69-82e2-e3eecc3d83d2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_14988e9f-f719-4135-84e0-d68f3155988c" xlink:to="loc_us-gaap_CommonClassAMember_1eb5f181-18e6-4f69-82e2-e3eecc3d83d2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_0546b528-ecdc-4717-bc3d-830f26f22708" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_14988e9f-f719-4135-84e0-d68f3155988c" xlink:to="loc_us-gaap_CommonClassBMember_0546b528-ecdc-4717-bc3d-830f26f22708" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/CommonStockandCommonStockWarrantsReservedforIssuanceDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#CommonStockandCommonStockWarrantsReservedforIssuanceDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/CommonStockandCommonStockWarrantsReservedforIssuanceDetails" xlink:type="extended" id="icb3ab85f0c424e2fa963f9d8769e7810_CommonStockandCommonStockWarrantsReservedforIssuanceDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_af8ce369-03c0-4e4c-886c-0cc8ec9ed219" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_03c82348-29dd-43e6-b721-5fadee6857c8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_af8ce369-03c0-4e4c-886c-0cc8ec9ed219" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_03c82348-29dd-43e6-b721-5fadee6857c8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_ad846eaf-9555-4f22-9d23-f40d6294c212" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_af8ce369-03c0-4e4c-886c-0cc8ec9ed219" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_ad846eaf-9555-4f22-9d23-f40d6294c212" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_51d430f9-2be0-4a6d-a697-34a6fc6561a4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_ad846eaf-9555-4f22-9d23-f40d6294c212" xlink:to="loc_us-gaap_AwardTypeAxis_51d430f9-2be0-4a6d-a697-34a6fc6561a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_51d430f9-2be0-4a6d-a697-34a6fc6561a4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_51d430f9-2be0-4a6d-a697-34a6fc6561a4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_51d430f9-2be0-4a6d-a697-34a6fc6561a4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_edc5bca7-394b-4452-9216-52efa2027950" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_51d430f9-2be0-4a6d-a697-34a6fc6561a4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_edc5bca7-394b-4452-9216-52efa2027950" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_012f9a2b-c36a-4932-909f-3f6a15480cd3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_edc5bca7-394b-4452-9216-52efa2027950" xlink:to="loc_us-gaap_EmployeeStockOptionMember_012f9a2b-c36a-4932-909f-3f6a15480cd3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockCompensationPlanMember_ca3c23c4-99f1-4515-8c6c-faf591ba42ab" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockCompensationPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_edc5bca7-394b-4452-9216-52efa2027950" xlink:to="loc_us-gaap_StockCompensationPlanMember_ca3c23c4-99f1-4515-8c6c-faf591ba42ab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_92100469-0f9c-494e-a11e-bb232f12f0dc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_edc5bca7-394b-4452-9216-52efa2027950" xlink:to="loc_us-gaap_EmployeeStockMember_92100469-0f9c-494e-a11e-bb232f12f0dc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_1dedf6c4-55bb-4093-970e-a431368ce507" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_edc5bca7-394b-4452-9216-52efa2027950" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_1dedf6c4-55bb-4093-970e-a431368ce507" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_DelayedShareIssuanceRelatedToAcquisitionMember_eccca47d-0c0a-4276-8600-817a7647f57a" xlink:href="tdup-20211231.xsd#tdup_DelayedShareIssuanceRelatedToAcquisitionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_edc5bca7-394b-4452-9216-52efa2027950" xlink:to="loc_tdup_DelayedShareIssuanceRelatedToAcquisitionMember_eccca47d-0c0a-4276-8600-817a7647f57a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertiblePreferredStockMember_76449cc3-2b85-4be9-8e43-62c44d957dc6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_edc5bca7-394b-4452-9216-52efa2027950" xlink:to="loc_us-gaap_ConvertiblePreferredStockMember_76449cc3-2b85-4be9-8e43-62c44d957dc6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_31a6bbe3-cacf-430b-9ae7-8cc711031f74" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_edc5bca7-394b-4452-9216-52efa2027950" xlink:to="loc_us-gaap_WarrantMember_31a6bbe3-cacf-430b-9ae7-8cc711031f74" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/StockBasedCompensationPlansNarrativeDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#StockBasedCompensationPlansNarrativeDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/StockBasedCompensationPlansNarrativeDetails" xlink:type="extended" id="i65cf9e104f3d454ea7c81ae5e63049f0_StockBasedCompensationPlansNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_d83ac2de-4eba-4cbb-8e24-3144ff0fa7ff" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_d83ac2de-4eba-4cbb-8e24-3144ff0fa7ff" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_b82b3453-83ae-40f4-b80d-ea8afa4e6baa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_b82b3453-83ae-40f4-b80d-ea8afa4e6baa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_215d7555-a25c-4142-a529-e831f4ac6704" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_215d7555-a25c-4142-a529-e831f4ac6704" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_6716890a-b66c-46b9-9238-c4ad7384caf0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_6716890a-b66c-46b9-9238-c4ad7384caf0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_0b7b41f2-e29c-4e56-83a3-907ced10c8a3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_0b7b41f2-e29c-4e56-83a3-907ced10c8a3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_424e79e0-d886-4770-acf6-1901c29f4fe3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_424e79e0-d886-4770-acf6-1901c29f4fe3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfTotalAwards_5c6ed779-6238-4781-9704-ffb2f7eeba91" xlink:href="tdup-20211231.xsd#tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfTotalAwards"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfTotalAwards_5c6ed779-6238-4781-9704-ffb2f7eeba91" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardPeriodAfterInitialPublicOfferingToCommenceVesting_1532cbef-0925-4cea-98c2-18da00a14982" xlink:href="tdup-20211231.xsd#tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardPeriodAfterInitialPublicOfferingToCommenceVesting"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardPeriodAfterInitialPublicOfferingToCommenceVesting_1532cbef-0925-4cea-98c2-18da00a14982" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_180c4e99-0f66-4c4a-9e80-01d71ec09e61" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_180c4e99-0f66-4c4a-9e80-01d71ec09e61" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_62087ca0-640e-41ea-b323-bc75e062f433" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_62087ca0-640e-41ea-b323-bc75e062f433" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_157b30c2-2594-4c63-9037-b7f43d421ea7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_157b30c2-2594-4c63-9037-b7f43d421ea7" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_9f61e69d-c1ef-46f3-b4ed-b2ae2b810b22" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_9f61e69d-c1ef-46f3-b4ed-b2ae2b810b22" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_01f77ded-3957-48a5-9097-a19af628f10a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_01f77ded-3957-48a5-9097-a19af628f10a" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConversionOfStockSharesConverted1_aa19d46a-fd64-4ece-a3a7-28ba937b1ff3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConversionOfStockSharesConverted1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_ConversionOfStockSharesConverted1_aa19d46a-fd64-4ece-a3a7-28ba937b1ff3" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_e754814e-9d1a-4160-b017-d4405b48fef3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_e754814e-9d1a-4160-b017-d4405b48fef3" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount_d5e94269-0427-4cf5-b446-23df6d73e17d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount_d5e94269-0427-4cf5-b446-23df6d73e17d" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_e49fb858-b0dc-4130-8af9-962e12072911" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_e49fb858-b0dc-4130-8af9-962e12072911" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_4a3cbf83-3d9a-4df7-9072-0a71063ab25a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_4a3cbf83-3d9a-4df7-9072-0a71063ab25a" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_34de16ca-56ee-4a5a-b0de-54c6f608c9c0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_34de16ca-56ee-4a5a-b0de-54c6f608c9c0" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested_7dfc7dfc-a295-46af-8360-b9f69a49070c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested_7dfc7dfc-a295-46af-8360-b9f69a49070c" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_ef491ebf-bd05-488a-9ca1-285536f4e7d3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_ef491ebf-bd05-488a-9ca1-285536f4e7d3" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_fe52dc43-fc2c-476a-bbdb-c74f28f728a5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5c6c3575-addb-43ce-9b87-01539fb0804e" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_fe52dc43-fc2c-476a-bbdb-c74f28f728a5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_09242666-6f45-4908-8a49-5a2106872741" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_fe52dc43-fc2c-476a-bbdb-c74f28f728a5" xlink:to="loc_us-gaap_AwardTypeAxis_09242666-6f45-4908-8a49-5a2106872741" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_09242666-6f45-4908-8a49-5a2106872741_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_09242666-6f45-4908-8a49-5a2106872741" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_09242666-6f45-4908-8a49-5a2106872741_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1d331c92-e115-4419-b49d-07e7e599c15d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_09242666-6f45-4908-8a49-5a2106872741" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1d331c92-e115-4419-b49d-07e7e599c15d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_c1bec90c-0354-409f-9bc1-da357383d0fc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1d331c92-e115-4419-b49d-07e7e599c15d" xlink:to="loc_us-gaap_EmployeeStockOptionMember_c1bec90c-0354-409f-9bc1-da357383d0fc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ShareBasedPaymentArrangementOptionVestingFourYearsFromCommencementMember_a7b887e2-e641-4497-a7da-2f5f144d828f" xlink:href="tdup-20211231.xsd#tdup_ShareBasedPaymentArrangementOptionVestingFourYearsFromCommencementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeStockOptionMember_c1bec90c-0354-409f-9bc1-da357383d0fc" xlink:to="loc_tdup_ShareBasedPaymentArrangementOptionVestingFourYearsFromCommencementMember_a7b887e2-e641-4497-a7da-2f5f144d828f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ShareBasedPaymentArrangementOptionFourYearVestingFromIPOMember_81f377c5-2db1-444d-a00b-69b8842f38db" xlink:href="tdup-20211231.xsd#tdup_ShareBasedPaymentArrangementOptionFourYearVestingFromIPOMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeStockOptionMember_c1bec90c-0354-409f-9bc1-da357383d0fc" xlink:to="loc_tdup_ShareBasedPaymentArrangementOptionFourYearVestingFromIPOMember_81f377c5-2db1-444d-a00b-69b8842f38db" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_381b5323-94c4-4afa-a6f5-d6767d0cbb6e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1d331c92-e115-4419-b49d-07e7e599c15d" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_381b5323-94c4-4afa-a6f5-d6767d0cbb6e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_9fbcf81c-1893-4486-a1ae-f8205dc3b0bb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1d331c92-e115-4419-b49d-07e7e599c15d" xlink:to="loc_us-gaap_EmployeeStockMember_9fbcf81c-1893-4486-a1ae-f8205dc3b0bb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_34db123e-6b0e-4385-b348-9624afc842f3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_fe52dc43-fc2c-476a-bbdb-c74f28f728a5" xlink:to="loc_us-gaap_PlanNameAxis_34db123e-6b0e-4385-b348-9624afc842f3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_34db123e-6b0e-4385-b348-9624afc842f3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_34db123e-6b0e-4385-b348-9624afc842f3" xlink:to="loc_us-gaap_PlanNameDomain_34db123e-6b0e-4385-b348-9624afc842f3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_ff06b19f-171e-4e77-af6c-d43cefbdbc53" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_34db123e-6b0e-4385-b348-9624afc842f3" xlink:to="loc_us-gaap_PlanNameDomain_ff06b19f-171e-4e77-af6c-d43cefbdbc53" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_StockIncentivePlan2010Member_2a1909bd-ab8d-49d4-b90b-c1002fb0543a" xlink:href="tdup-20211231.xsd#tdup_StockIncentivePlan2010Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_ff06b19f-171e-4e77-af6c-d43cefbdbc53" xlink:to="loc_tdup_StockIncentivePlan2010Member_2a1909bd-ab8d-49d4-b90b-c1002fb0543a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_StockIncentivePlan2021Member_5f06da70-bcd0-4f9f-9372-552549f07d10" xlink:href="tdup-20211231.xsd#tdup_StockIncentivePlan2021Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_ff06b19f-171e-4e77-af6c-d43cefbdbc53" xlink:to="loc_tdup_StockIncentivePlan2021Member_5f06da70-bcd0-4f9f-9372-552549f07d10" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_382e4b41-0e4a-414e-909e-53fa8223e83b" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_fe52dc43-fc2c-476a-bbdb-c74f28f728a5" xlink:to="loc_srt_TitleOfIndividualAxis_382e4b41-0e4a-414e-909e-53fa8223e83b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_382e4b41-0e4a-414e-909e-53fa8223e83b_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_382e4b41-0e4a-414e-909e-53fa8223e83b" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_382e4b41-0e4a-414e-909e-53fa8223e83b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_f07b574b-14b8-4ea7-ab1a-5bb12cd64598" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_382e4b41-0e4a-414e-909e-53fa8223e83b" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_f07b574b-14b8-4ea7-ab1a-5bb12cd64598" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_Over10PercentShareholderMember_7910a062-636d-45c4-915e-800247fe1f63" xlink:href="tdup-20211231.xsd#tdup_Over10PercentShareholderMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_f07b574b-14b8-4ea7-ab1a-5bb12cd64598" xlink:to="loc_tdup_Over10PercentShareholderMember_7910a062-636d-45c4-915e-800247fe1f63" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/StockBasedCompensationPlansScheduleofStockOptionActivityDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#StockBasedCompensationPlansScheduleofStockOptionActivityDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/StockBasedCompensationPlansScheduleofStockOptionActivityDetails" xlink:type="extended" id="icc3d549802a246d68e85658072fa07b1_StockBasedCompensationPlansScheduleofStockOptionActivityDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0a6f3e4b-3806-44a7-ad4d-16c8b320fbf6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_a67f8d98-8f08-44e0-9da3-bdd9706ba0e3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0a6f3e4b-3806-44a7-ad4d-16c8b320fbf6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_a67f8d98-8f08-44e0-9da3-bdd9706ba0e3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_f28a2270-ef0b-496d-b58b-26dd7fd95865" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_a67f8d98-8f08-44e0-9da3-bdd9706ba0e3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_f28a2270-ef0b-496d-b58b-26dd7fd95865" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_515eaf7c-8a79-432e-90c2-3732d3f73a5f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_a67f8d98-8f08-44e0-9da3-bdd9706ba0e3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_515eaf7c-8a79-432e-90c2-3732d3f73a5f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_71391a75-b264-4a50-86be-700fc5655fcd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_a67f8d98-8f08-44e0-9da3-bdd9706ba0e3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_71391a75-b264-4a50-86be-700fc5655fcd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_aa3c1755-a40b-4b73-9c11-c38637ad2fb7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_a67f8d98-8f08-44e0-9da3-bdd9706ba0e3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_aa3c1755-a40b-4b73-9c11-c38637ad2fb7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_b252e894-ea93-498e-be6d-01bbe982b93e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_a67f8d98-8f08-44e0-9da3-bdd9706ba0e3" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_b252e894-ea93-498e-be6d-01bbe982b93e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_776ebe75-68a2-4ad6-815f-d6eb0b68d3eb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_a67f8d98-8f08-44e0-9da3-bdd9706ba0e3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_776ebe75-68a2-4ad6-815f-d6eb0b68d3eb" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod_7d8553b1-9798-440d-8770-823f33c06690" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_a67f8d98-8f08-44e0-9da3-bdd9706ba0e3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod_7d8553b1-9798-440d-8770-823f33c06690" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsTransferred_75879137-f6bb-4a39-a86e-96319ced122d" xlink:href="tdup-20211231.xsd#tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsTransferred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_a67f8d98-8f08-44e0-9da3-bdd9706ba0e3" xlink:to="loc_tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsTransferred_75879137-f6bb-4a39-a86e-96319ced122d" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_c8cc4a64-2612-4209-a2f1-70907cb4915f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_7903cf7f-6203-43c6-93bd-d3c28f71d223" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_d70fefc6-6edb-4aaf-ad60-493eafe970fe" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0a6f3e4b-3806-44a7-ad4d-16c8b320fbf6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_d70fefc6-6edb-4aaf-ad60-493eafe970fe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_19939401-548a-483f-b66e-30c1307b04b3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0a6f3e4b-3806-44a7-ad4d-16c8b320fbf6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_19939401-548a-483f-b66e-30c1307b04b3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_20d01a68-7088-41ba-a172-df8669aa49bc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_19939401-548a-483f-b66e-30c1307b04b3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_20d01a68-7088-41ba-a172-df8669aa49bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_7747d8aa-f47f-4a38-93d2-1355e04157d3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_19939401-548a-483f-b66e-30c1307b04b3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_7747d8aa-f47f-4a38-93d2-1355e04157d3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_234e1455-cc9a-4ae5-9bec-e862af7f0102" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_19939401-548a-483f-b66e-30c1307b04b3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_234e1455-cc9a-4ae5-9bec-e862af7f0102" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_96327c54-be23-49ac-ad19-916fba7f39bc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_19939401-548a-483f-b66e-30c1307b04b3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_96327c54-be23-49ac-ad19-916fba7f39bc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_c18239a8-44d1-4483-88f5-bcf088493543" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_1d7e957a-d514-492d-b4f2-be16b0fc98ea" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0a6f3e4b-3806-44a7-ad4d-16c8b320fbf6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_1d7e957a-d514-492d-b4f2-be16b0fc98ea" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract_4aec8caf-a0e6-4bcb-b395-2fd937ad7def" xlink:href="tdup-20211231.xsd#tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0a6f3e4b-3806-44a7-ad4d-16c8b320fbf6" xlink:to="loc_tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract_4aec8caf-a0e6-4bcb-b395-2fd937ad7def" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_c16e2736-8c84-45f2-ab2c-4e86d2634e5e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract_4aec8caf-a0e6-4bcb-b395-2fd937ad7def" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_c16e2736-8c84-45f2-ab2c-4e86d2634e5e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_bd806d7d-1add-4837-96c4-45836c939cc4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract_4aec8caf-a0e6-4bcb-b395-2fd937ad7def" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_bd806d7d-1add-4837-96c4-45836c939cc4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_b28ea9d9-bf17-45a9-8688-3f2180d8c158" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0a6f3e4b-3806-44a7-ad4d-16c8b320fbf6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_b28ea9d9-bf17-45a9-8688-3f2180d8c158" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_de8d7da3-e210-4992-8f92-ce6b16eb61b8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_b28ea9d9-bf17-45a9-8688-3f2180d8c158" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_de8d7da3-e210-4992-8f92-ce6b16eb61b8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_bc813bcd-5aa3-438f-a317-743c7a473714" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_b28ea9d9-bf17-45a9-8688-3f2180d8c158" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_bc813bcd-5aa3-438f-a317-743c7a473714" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_974621bf-928d-4c85-914a-e3ca828ff746" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0a6f3e4b-3806-44a7-ad4d-16c8b320fbf6" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_974621bf-928d-4c85-914a-e3ca828ff746" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_ea21191e-1848-44cb-b7be-341d347c98ac" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_974621bf-928d-4c85-914a-e3ca828ff746" xlink:to="loc_us-gaap_AwardTypeAxis_ea21191e-1848-44cb-b7be-341d347c98ac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ea21191e-1848-44cb-b7be-341d347c98ac_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_ea21191e-1848-44cb-b7be-341d347c98ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ea21191e-1848-44cb-b7be-341d347c98ac_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f780a36c-b800-47e6-b269-53e29e3f6975" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_ea21191e-1848-44cb-b7be-341d347c98ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f780a36c-b800-47e6-b269-53e29e3f6975" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_d425a76e-98ae-43f0-9def-9f8fac45953d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f780a36c-b800-47e6-b269-53e29e3f6975" xlink:to="loc_us-gaap_EmployeeStockOptionMember_d425a76e-98ae-43f0-9def-9f8fac45953d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_20c3bba0-4224-4e50-a11e-b5a3fd0c898e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_974621bf-928d-4c85-914a-e3ca828ff746" xlink:to="loc_us-gaap_PlanNameAxis_20c3bba0-4224-4e50-a11e-b5a3fd0c898e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_20c3bba0-4224-4e50-a11e-b5a3fd0c898e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_20c3bba0-4224-4e50-a11e-b5a3fd0c898e" xlink:to="loc_us-gaap_PlanNameDomain_20c3bba0-4224-4e50-a11e-b5a3fd0c898e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_ac82a45f-2834-4d27-8c17-cb54495d464c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_20c3bba0-4224-4e50-a11e-b5a3fd0c898e" xlink:to="loc_us-gaap_PlanNameDomain_ac82a45f-2834-4d27-8c17-cb54495d464c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_StockIncentivePlan2010Member_f98d7a44-0a89-44c2-988d-1281ae65c326" xlink:href="tdup-20211231.xsd#tdup_StockIncentivePlan2010Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_ac82a45f-2834-4d27-8c17-cb54495d464c" xlink:to="loc_tdup_StockIncentivePlan2010Member_f98d7a44-0a89-44c2-988d-1281ae65c326" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/StockBasedCompensationPlansScheduleofFairValueAssumptionsDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#StockBasedCompensationPlansScheduleofFairValueAssumptionsDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/StockBasedCompensationPlansScheduleofFairValueAssumptionsDetails" xlink:type="extended" id="if23621417cc345a6b89a909f4fa6315e_StockBasedCompensationPlansScheduleofFairValueAssumptionsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b765110a-208c-46e4-b13d-8fd1f80321d7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_3abc9e8f-9ad4-4eb5-a729-03c1a89013fd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b765110a-208c-46e4-b13d-8fd1f80321d7" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_3abc9e8f-9ad4-4eb5-a729-03c1a89013fd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum_dbe7f571-d9bc-4bfe-a9b9-a1dc99173e44" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b765110a-208c-46e4-b13d-8fd1f80321d7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum_dbe7f571-d9bc-4bfe-a9b9-a1dc99173e44" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum_b06aab66-3a33-452a-9f18-d4afb7e63804" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b765110a-208c-46e4-b13d-8fd1f80321d7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum_b06aab66-3a33-452a-9f18-d4afb7e63804" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum_ed58b5e3-6e38-493b-83f1-a25b8db8b3ef" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b765110a-208c-46e4-b13d-8fd1f80321d7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum_ed58b5e3-6e38-493b-83f1-a25b8db8b3ef" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum_15ff6170-a2a3-4511-b299-ee2c2acc4dc0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b765110a-208c-46e4-b13d-8fd1f80321d7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum_15ff6170-a2a3-4511-b299-ee2c2acc4dc0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_2d569271-d8c7-4c40-b391-ef5bc1ac76a5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b765110a-208c-46e4-b13d-8fd1f80321d7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_2d569271-d8c7-4c40-b391-ef5bc1ac76a5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExercisePrice_73094695-6375-4925-9a94-f779f8cd3187" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b765110a-208c-46e4-b13d-8fd1f80321d7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExercisePrice_73094695-6375-4925-9a94-f779f8cd3187" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5bfb92cd-c693-44a6-b126-b1418bc2d45e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b765110a-208c-46e4-b13d-8fd1f80321d7" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5bfb92cd-c693-44a6-b126-b1418bc2d45e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_8163f839-8dfc-4867-a7ac-3a2ee828b5e7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5bfb92cd-c693-44a6-b126-b1418bc2d45e" xlink:to="loc_us-gaap_AwardTypeAxis_8163f839-8dfc-4867-a7ac-3a2ee828b5e7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8163f839-8dfc-4867-a7ac-3a2ee828b5e7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_8163f839-8dfc-4867-a7ac-3a2ee828b5e7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8163f839-8dfc-4867-a7ac-3a2ee828b5e7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4af39453-4a7f-4418-82ac-3d272d768f79" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_8163f839-8dfc-4867-a7ac-3a2ee828b5e7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4af39453-4a7f-4418-82ac-3d272d768f79" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_9c7d84e7-fde1-4908-b719-db0dc1952476" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4af39453-4a7f-4418-82ac-3d272d768f79" xlink:to="loc_us-gaap_EmployeeStockOptionMember_9c7d84e7-fde1-4908-b719-db0dc1952476" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_d68e4ccd-d747-4556-b6c0-9377070a5ba5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4af39453-4a7f-4418-82ac-3d272d768f79" xlink:to="loc_us-gaap_EmployeeStockMember_d68e4ccd-d747-4556-b6c0-9377070a5ba5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_564bd36b-a9ff-4bf2-85d8-fdf361eea034" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5bfb92cd-c693-44a6-b126-b1418bc2d45e" xlink:to="loc_srt_RangeAxis_564bd36b-a9ff-4bf2-85d8-fdf361eea034" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_564bd36b-a9ff-4bf2-85d8-fdf361eea034_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_564bd36b-a9ff-4bf2-85d8-fdf361eea034" xlink:to="loc_srt_RangeMember_564bd36b-a9ff-4bf2-85d8-fdf361eea034_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_32ec98fa-10ef-475f-90e9-234351d866f8" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_564bd36b-a9ff-4bf2-85d8-fdf361eea034" xlink:to="loc_srt_RangeMember_32ec98fa-10ef-475f-90e9-234351d866f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_faf4a980-c178-41e4-9bee-61eedeea8448" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_32ec98fa-10ef-475f-90e9-234351d866f8" xlink:to="loc_srt_MinimumMember_faf4a980-c178-41e4-9bee-61eedeea8448" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_2ccbc12e-9380-4d9d-8c38-e934ca050746" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_32ec98fa-10ef-475f-90e9-234351d866f8" xlink:to="loc_srt_MaximumMember_2ccbc12e-9380-4d9d-8c38-e934ca050746" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/StockBasedCompensationPlansScheduleofRSUActivityDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#StockBasedCompensationPlansScheduleofRSUActivityDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/StockBasedCompensationPlansScheduleofRSUActivityDetails" xlink:type="extended" id="i27d63ad0c4094b25bc44af3b5996ed79_StockBasedCompensationPlansScheduleofRSUActivityDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0dce1129-3989-40e9-8229-5468e168a513" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_4e9c1293-a9f9-4ffa-aa74-99333d7a0ef9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0dce1129-3989-40e9-8229-5468e168a513" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_4e9c1293-a9f9-4ffa-aa74-99333d7a0ef9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_8d3893c2-5e74-4fd4-83da-9dadadcac773" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_4e9c1293-a9f9-4ffa-aa74-99333d7a0ef9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_8d3893c2-5e74-4fd4-83da-9dadadcac773" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_2cf7b5f5-1556-49a6-880a-3b1399ce3e2e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_4e9c1293-a9f9-4ffa-aa74-99333d7a0ef9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_2cf7b5f5-1556-49a6-880a-3b1399ce3e2e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_60aa88d9-bb6e-40a9-bddc-5ddba7e74980" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_4e9c1293-a9f9-4ffa-aa74-99333d7a0ef9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_60aa88d9-bb6e-40a9-bddc-5ddba7e74980" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_a3943548-2b42-4ca2-881c-1d79a7cf6713" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_4e9c1293-a9f9-4ffa-aa74-99333d7a0ef9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_a3943548-2b42-4ca2-881c-1d79a7cf6713" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_8b868cd3-b551-4b81-b925-c0ddff724e4b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_150a0b4b-6f7a-4f1f-a8e3-deef840925b8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0dce1129-3989-40e9-8229-5468e168a513" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_150a0b4b-6f7a-4f1f-a8e3-deef840925b8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_404f4226-df11-484c-8abe-f252dd20c083" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_150a0b4b-6f7a-4f1f-a8e3-deef840925b8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_404f4226-df11-484c-8abe-f252dd20c083" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_6073cc41-81e9-4926-a5bb-55879b2fe792" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_150a0b4b-6f7a-4f1f-a8e3-deef840925b8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_6073cc41-81e9-4926-a5bb-55879b2fe792" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_89afbb26-4231-4182-b28e-7f48f491a1aa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_150a0b4b-6f7a-4f1f-a8e3-deef840925b8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_89afbb26-4231-4182-b28e-7f48f491a1aa" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_1b209922-1548-4afa-8e57-5589a4bca87d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_150a0b4b-6f7a-4f1f-a8e3-deef840925b8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_1b209922-1548-4afa-8e57-5589a4bca87d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_40cd31af-70a7-4fdb-bc82-1c1599a5a016" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms_fb4e41eb-0a48-4519-9639-23e420621c03" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0dce1129-3989-40e9-8229-5468e168a513" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms_fb4e41eb-0a48-4519-9639-23e420621c03" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding_a164a8d6-59d2-4891-b19b-68a340d1f37b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0dce1129-3989-40e9-8229-5468e168a513" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding_a164a8d6-59d2-4891-b19b-68a340d1f37b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_1c5a40cf-50ea-4e22-ac9b-0a43fb15c8d8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0dce1129-3989-40e9-8229-5468e168a513" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_1c5a40cf-50ea-4e22-ac9b-0a43fb15c8d8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_5145daf0-e12d-4ceb-9333-a63a452106ba" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_1c5a40cf-50ea-4e22-ac9b-0a43fb15c8d8" xlink:to="loc_us-gaap_AwardTypeAxis_5145daf0-e12d-4ceb-9333-a63a452106ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5145daf0-e12d-4ceb-9333-a63a452106ba_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_5145daf0-e12d-4ceb-9333-a63a452106ba" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5145daf0-e12d-4ceb-9333-a63a452106ba_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_eea16e76-c53a-4f2d-9f83-23367aa64600" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_5145daf0-e12d-4ceb-9333-a63a452106ba" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_eea16e76-c53a-4f2d-9f83-23367aa64600" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_4f72728a-b5be-44a1-9dc2-d9a5d01a2e26" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_eea16e76-c53a-4f2d-9f83-23367aa64600" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_4f72728a-b5be-44a1-9dc2-d9a5d01a2e26" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/StockBasedCompensationPlansScheduleofStockBasedCompensationExpenseDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#StockBasedCompensationPlansScheduleofStockBasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/StockBasedCompensationPlansScheduleofStockBasedCompensationExpenseDetails" xlink:type="extended" id="ica6c315e201f4a04bbfa9322f921d57e_StockBasedCompensationPlansScheduleofStockBasedCompensationExpenseDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5943dcb9-a97a-48a1-a00e-2bf49391e18d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_2bb5ceb5-50e6-4f15-9dfd-696f794f6a66" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5943dcb9-a97a-48a1-a00e-2bf49391e18d" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_2bb5ceb5-50e6-4f15-9dfd-696f794f6a66" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_3532a0e4-1c85-4536-8b42-b2e342364546" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5943dcb9-a97a-48a1-a00e-2bf49391e18d" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_3532a0e4-1c85-4536-8b42-b2e342364546" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_76293eb8-200f-4fd1-aa65-136ef1b6d847" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_3532a0e4-1c85-4536-8b42-b2e342364546" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_76293eb8-200f-4fd1-aa65-136ef1b6d847" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_76293eb8-200f-4fd1-aa65-136ef1b6d847_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_76293eb8-200f-4fd1-aa65-136ef1b6d847" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_76293eb8-200f-4fd1-aa65-136ef1b6d847_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_c9a7cbc2-bcc2-40f9-85b7-a4ee142318e3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_76293eb8-200f-4fd1-aa65-136ef1b6d847" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_c9a7cbc2-bcc2-40f9-85b7-a4ee142318e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_OperationsProductAndTechnologyMember_948c0862-00ec-451f-8ab9-6a9613b22975" xlink:href="tdup-20211231.xsd#tdup_OperationsProductAndTechnologyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_c9a7cbc2-bcc2-40f9-85b7-a4ee142318e3" xlink:to="loc_tdup_OperationsProductAndTechnologyMember_948c0862-00ec-451f-8ab9-6a9613b22975" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_MarketingExpenseMember_62b74fda-bc5d-4f4b-a83a-737357b32d76" xlink:href="tdup-20211231.xsd#tdup_MarketingExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_c9a7cbc2-bcc2-40f9-85b7-a4ee142318e3" xlink:to="loc_tdup_MarketingExpenseMember_62b74fda-bc5d-4f4b-a83a-737357b32d76" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_d29ffba0-a3c6-4274-b594-c663bc9dfe82" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_c9a7cbc2-bcc2-40f9-85b7-a4ee142318e3" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_d29ffba0-a3c6-4274-b594-c663bc9dfe82" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/IncomeTaxesNarrativeDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#IncomeTaxesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/IncomeTaxesNarrativeDetails" xlink:type="extended" id="i6c84fc8cbe6a43aab1d4690581676636_IncomeTaxesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxContingencyLineItems_4f2a06f5-8767-4911-8260-099cc6500edc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxContingencyLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_24b800e1-fa6a-41cf-9c8c-afe391c156f0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_4f2a06f5-8767-4911-8260-099cc6500edc" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_24b800e1-fa6a-41cf-9c8c-afe391c156f0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_7c838067-2de7-448e-b4b2-4991d938b8f7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_4f2a06f5-8767-4911-8260-099cc6500edc" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_7c838067-2de7-448e-b4b2-4991d938b8f7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwards_e35313c1-4908-4553-bf0d-d9e4a3eddc14" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwards"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_4f2a06f5-8767-4911-8260-099cc6500edc" xlink:to="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwards_e35313c1-4908-4553-bf0d-d9e4a3eddc14" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount_4be7274a-f02f-48fd-a70a-fc095216a428" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_4f2a06f5-8767-4911-8260-099cc6500edc" xlink:to="loc_us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount_4be7274a-f02f-48fd-a70a-fc095216a428" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwards_27183f8b-ac7d-4bf7-b70f-2424fca508fa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLossCarryforwards"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_4f2a06f5-8767-4911-8260-099cc6500edc" xlink:to="loc_us-gaap_OperatingLossCarryforwards_27183f8b-ac7d-4bf7-b70f-2424fca508fa" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration_4709fcec-c15d-4814-859a-e1525b7bead2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_4f2a06f5-8767-4911-8260-099cc6500edc" xlink:to="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration_4709fcec-c15d-4814-859a-e1525b7bead2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefits_2b51a49f-8f38-46c7-9e2c-415c0807ea8a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_UnrecognizedTaxBenefits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_4f2a06f5-8767-4911-8260-099cc6500edc" xlink:to="loc_us-gaap_UnrecognizedTaxBenefits_2b51a49f-8f38-46c7-9e2c-415c0807ea8a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued_1c143453-8e7d-4d68-b080-612506510fa1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_4f2a06f5-8767-4911-8260-099cc6500edc" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued_1c143453-8e7d-4d68-b080-612506510fa1" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxContingencyTable_a4a8e44d-010e-4232-bbcd-1c0fa23eb038" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxContingencyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_4f2a06f5-8767-4911-8260-099cc6500edc" xlink:to="loc_us-gaap_IncomeTaxContingencyTable_a4a8e44d-010e-4232-bbcd-1c0fa23eb038" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_f65c19cc-8b98-41cf-91fd-243f9cd36507" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxAuthorityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeTaxContingencyTable_a4a8e44d-010e-4232-bbcd-1c0fa23eb038" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_f65c19cc-8b98-41cf-91fd-243f9cd36507" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_f65c19cc-8b98-41cf-91fd-243f9cd36507_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_f65c19cc-8b98-41cf-91fd-243f9cd36507" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_f65c19cc-8b98-41cf-91fd-243f9cd36507_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_a9276273-0978-4b87-bd05-9fca365060d6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_f65c19cc-8b98-41cf-91fd-243f9cd36507" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_a9276273-0978-4b87-bd05-9fca365060d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticCountryMember_e075534e-035c-469b-9428-a2b3a1c1f751" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DomesticCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_a9276273-0978-4b87-bd05-9fca365060d6" xlink:to="loc_us-gaap_DomesticCountryMember_e075534e-035c-469b-9428-a2b3a1c1f751" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCountryMember_e4870b59-5a21-46e3-a4dc-64d496183f3b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ForeignCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_a9276273-0978-4b87-bd05-9fca365060d6" xlink:to="loc_us-gaap_ForeignCountryMember_e4870b59-5a21-46e3-a4dc-64d496183f3b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StateAndLocalJurisdictionMember_8b926bab-ec93-4617-9ec1-70291771c083" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StateAndLocalJurisdictionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_a9276273-0978-4b87-bd05-9fca365060d6" xlink:to="loc_us-gaap_StateAndLocalJurisdictionMember_8b926bab-ec93-4617-9ec1-70291771c083" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/NetLossPerShareAttributabletoCommonStockholdersDetails" xlink:type="simple" xlink:href="tdup-20211231.xsd#NetLossPerShareAttributabletoCommonStockholdersDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/NetLossPerShareAttributabletoCommonStockholdersDetails" xlink:type="extended" id="iaa303851861247fba215b13136eb7303_NetLossPerShareAttributabletoCommonStockholdersDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_0b6f62bc-499a-4ff8-a349-fff94587ba14" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_0bac1df7-0b50-47ca-8f50-512b3e4c32ac" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_0b6f62bc-499a-4ff8-a349-fff94587ba14" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_0bac1df7-0b50-47ca-8f50-512b3e4c32ac" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_8545da6c-f178-4da4-8f08-a7164090c2f6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_0b6f62bc-499a-4ff8-a349-fff94587ba14" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_8545da6c-f178-4da4-8f08-a7164090c2f6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_c9fca8e0-4073-4d76-8257-3bdd2ef5507e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_8545da6c-f178-4da4-8f08-a7164090c2f6" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_c9fca8e0-4073-4d76-8257-3bdd2ef5507e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_c9fca8e0-4073-4d76-8257-3bdd2ef5507e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_c9fca8e0-4073-4d76-8257-3bdd2ef5507e" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_c9fca8e0-4073-4d76-8257-3bdd2ef5507e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_52524394-b758-4051-a7cd-994ecdb64a3c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_c9fca8e0-4073-4d76-8257-3bdd2ef5507e" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_52524394-b758-4051-a7cd-994ecdb64a3c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_da0f892b-d443-4d44-b21f-9ec9d046cf67" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_52524394-b758-4051-a7cd-994ecdb64a3c" xlink:to="loc_us-gaap_EmployeeStockOptionMember_da0f892b-d443-4d44-b21f-9ec9d046cf67" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_ec09a15b-2986-4540-b498-a9a59c350188" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_52524394-b758-4051-a7cd-994ecdb64a3c" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_ec09a15b-2986-4540-b498-a9a59c350188" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_DelayedShareIssuanceRelatedToAcquisitionMember_2237fdb1-0599-437f-a94f-6b064fcef2dc" xlink:href="tdup-20211231.xsd#tdup_DelayedShareIssuanceRelatedToAcquisitionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_52524394-b758-4051-a7cd-994ecdb64a3c" xlink:to="loc_tdup_DelayedShareIssuanceRelatedToAcquisitionMember_2237fdb1-0599-437f-a94f-6b064fcef2dc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_71550deb-b9b5-426a-97d8-2d0f3c38c583" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_52524394-b758-4051-a7cd-994ecdb64a3c" xlink:to="loc_us-gaap_EmployeeStockMember_71550deb-b9b5-426a-97d8-2d0f3c38c583" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertiblePreferredStockMember_284f0b2a-d9a3-46ec-a79f-5d5fba82125b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_52524394-b758-4051-a7cd-994ecdb64a3c" xlink:to="loc_us-gaap_ConvertiblePreferredStockMember_284f0b2a-d9a3-46ec-a79f-5d5fba82125b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_95e32a93-274e-477c-ad1d-4b9ab56c9fdb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_52524394-b758-4051-a7cd-994ecdb64a3c" xlink:to="loc_us-gaap_WarrantMember_95e32a93-274e-477c-ad1d-4b9ab56c9fdb" xlink:type="arc" order="5"/>
  </link:definitionLink>
</link:linkbase>
