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

<!--XBRL Document Created with Wdesk from Workiva-->
<!--Copyright 2021 Workiva-->
<!--r:0c2d5397-e633-4b70-af24-4066b33ee8cc,g:86c1e5ab-8226-4449-b09a-2105c4412d9c-->
<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-20210930.xsd#CoverPage"/>
  <link:definitionLink xlink:role="http://thredup.com/role/CoverPage" xlink:type="extended" id="i1edcf2dbe8b24968ab884d2f87774108_CoverPage">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_b5106a62-aca3-45c9-aa6a-e2a3f12b216c" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_DocumentType_b5106a62-aca3-45c9-aa6a-e2a3f12b216c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentQuarterlyReport_c3794bf9-d4c4-45b5-a71c-87f20d4fd3e6" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_DocumentQuarterlyReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_DocumentQuarterlyReport_c3794bf9-d4c4-45b5-a71c-87f20d4fd3e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_9396c922-b391-4b1a-b803-6646ccfc845c" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_DocumentPeriodEndDate_9396c922-b391-4b1a-b803-6646ccfc845c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_79c9e9e1-be7a-4b78-af02-de3a9eaef600" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_DocumentTransitionReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_DocumentTransitionReport_79c9e9e1-be7a-4b78-af02-de3a9eaef600" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_80564974-e7a3-4ebf-b733-3e54f2ec9d9e" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_EntityFileNumber_80564974-e7a3-4ebf-b733-3e54f2ec9d9e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_1c6c6eb3-59ea-43fe-99d1-2979c5bc6790" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_EntityRegistrantName_1c6c6eb3-59ea-43fe-99d1-2979c5bc6790" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_4ceaaeab-751a-4f0e-a873-df9d8bf48074" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_EntityIncorporationStateCountryCode_4ceaaeab-751a-4f0e-a873-df9d8bf48074" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_7b48734e-ecbd-48c4-88ba-1ba27ba4015a" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_EntityTaxIdentificationNumber_7b48734e-ecbd-48c4-88ba-1ba27ba4015a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_284cbdb7-a5a4-439e-a680-feca97d6c00d" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_EntityAddressAddressLine1_284cbdb7-a5a4-439e-a680-feca97d6c00d" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine2_514597b2-eef3-450d-997a-e833990557d8" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityAddressAddressLine2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_EntityAddressAddressLine2_514597b2-eef3-450d-997a-e833990557d8" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_73774663-faa9-431f-8a2f-b2cae9cdfbe7" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_EntityAddressCityOrTown_73774663-faa9-431f-8a2f-b2cae9cdfbe7" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_880d524e-630b-40a6-b2ec-e92aaa4df308" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_EntityAddressStateOrProvince_880d524e-630b-40a6-b2ec-e92aaa4df308" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_c2665740-890f-46d2-8279-cee8a6ff6bef" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_EntityAddressPostalZipCode_c2665740-890f-46d2-8279-cee8a6ff6bef" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_5d4f195d-8a6e-40c2-b3d1-ed377f53c201" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_CityAreaCode_5d4f195d-8a6e-40c2-b3d1-ed377f53c201" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_ec326d0c-5d87-4914-baf2-662bae41c1a3" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_LocalPhoneNumber_ec326d0c-5d87-4914-baf2-662bae41c1a3" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_42c6cf27-c61f-403a-b0fe-0ac8679d230c" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_Security12bTitle_42c6cf27-c61f-403a-b0fe-0ac8679d230c" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_522cf8b8-ccab-4965-9733-9ab76c3e4c87" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_TradingSymbol_522cf8b8-ccab-4965-9733-9ab76c3e4c87" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_4d048bd3-d829-4255-ac9e-df9acfd2f314" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_SecurityExchangeName_4d048bd3-d829-4255-ac9e-df9acfd2f314" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_001d0fa1-fc93-4630-b0ba-9752dd8708c4" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityCurrentReportingStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_EntityCurrentReportingStatus_001d0fa1-fc93-4630-b0ba-9752dd8708c4" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_1a9842ee-11e4-48c4-ae46-3004aa08016e" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityInteractiveDataCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_EntityInteractiveDataCurrent_1a9842ee-11e4-48c4-ae46-3004aa08016e" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_95d0d259-3917-449a-bef1-3fbe49fb07c3" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityFilerCategory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_EntityFilerCategory_95d0d259-3917-449a-bef1-3fbe49fb07c3" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_2ab7ae08-80d1-49e2-8d85-fa06291f0f2f" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntitySmallBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_EntitySmallBusiness_2ab7ae08-80d1-49e2-8d85-fa06291f0f2f" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_9c8da3ec-b462-443b-be92-7990bbe8f9cf" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_EntityEmergingGrowthCompany_9c8da3ec-b462-443b-be92-7990bbe8f9cf" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityExTransitionPeriod_b89cbcd9-7b2e-4224-aea9-5da6ad174124" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityExTransitionPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_EntityExTransitionPeriod_b89cbcd9-7b2e-4224-aea9-5da6ad174124" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_72edce3e-1a06-4cc2-b084-93e6c3f264ac" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityShellCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_EntityShellCompany_72edce3e-1a06-4cc2-b084-93e6c3f264ac" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_197453ca-a3fb-4e44-ae46-a64bcfb931f2" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityCommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_197453ca-a3fb-4e44-ae46-a64bcfb931f2" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_b079afb0-f7a6-4800-9850-8bbb9fa2056a" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_AmendmentFlag_b079afb0-f7a6-4800-9850-8bbb9fa2056a" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_cced9f16-d925-4e56-9e74-d68f47b36d4d" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_DocumentFiscalYearFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_DocumentFiscalYearFocus_cced9f16-d925-4e56-9e74-d68f47b36d4d" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_83324041-4727-4231-b290-59c870024941" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_DocumentFiscalPeriodFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_DocumentFiscalPeriodFocus_83324041-4727-4231-b290-59c870024941" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_59f195b5-9fa3-4b0e-bae3-9a2195c99dff" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_EntityCentralIndexKey_59f195b5-9fa3-4b0e-bae3-9a2195c99dff" xlink:type="arc" order="29"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_c78a7dac-1afd-4b98-8355-252634ac4012" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_CurrentFiscalYearEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_CurrentFiscalYearEndDate_c78a7dac-1afd-4b98-8355-252634ac4012" xlink:type="arc" order="30"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_fc3c8c1d-0bac-4c1b-9c0d-8ab2ace9c8a8" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_DocumentInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_978fa5ac-8e92-4958-bf0c-ffcf6ae5a89c" xlink:to="loc_dei_DocumentInformationTable_fc3c8c1d-0bac-4c1b-9c0d-8ab2ace9c8a8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_02c2086a-f2a2-4cd0-a611-e4a48353d942" 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_fc3c8c1d-0bac-4c1b-9c0d-8ab2ace9c8a8" xlink:to="loc_us-gaap_StatementClassOfStockAxis_02c2086a-f2a2-4cd0-a611-e4a48353d942" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_02c2086a-f2a2-4cd0-a611-e4a48353d942_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_02c2086a-f2a2-4cd0-a611-e4a48353d942" xlink:to="loc_us-gaap_ClassOfStockDomain_02c2086a-f2a2-4cd0-a611-e4a48353d942_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_1b5abdfb-0cd3-4831-818e-2597af362320" 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_02c2086a-f2a2-4cd0-a611-e4a48353d942" xlink:to="loc_us-gaap_ClassOfStockDomain_1b5abdfb-0cd3-4831-818e-2597af362320" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_dbab6505-af90-4d7e-be50-e2e2b57f5d0d" 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_1b5abdfb-0cd3-4831-818e-2597af362320" xlink:to="loc_us-gaap_CommonClassAMember_dbab6505-af90-4d7e-be50-e2e2b57f5d0d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_4be83915-60b3-41ad-8860-c44b764364e8" 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_1b5abdfb-0cd3-4831-818e-2597af362320" xlink:to="loc_us-gaap_CommonClassBMember_4be83915-60b3-41ad-8860-c44b764364e8" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/CondensedConsolidatedStatementsofOperationsandComprehensiveLoss" xlink:type="simple" xlink:href="tdup-20210930.xsd#CondensedConsolidatedStatementsofOperationsandComprehensiveLoss"/>
  <link:definitionLink xlink:role="http://thredup.com/role/CondensedConsolidatedStatementsofOperationsandComprehensiveLoss" xlink:type="extended" id="i2f5ac6acc424475aa482049c337e6404_CondensedConsolidatedStatementsofOperationsandComprehensiveLoss">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_8f5b7ebe-398c-4617-9dab-28b390741ee0" 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_483b631e-6ee5-468b-bb89-c93c9aee76f0" 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_8f5b7ebe-398c-4617-9dab-28b390741ee0" xlink:to="loc_us-gaap_RevenuesAbstract_483b631e-6ee5-468b-bb89-c93c9aee76f0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_7b0c5e4a-dbd0-4c32-b59b-8ee1e6a88ff4" 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_483b631e-6ee5-468b-bb89-c93c9aee76f0" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_7b0c5e4a-dbd0-4c32-b59b-8ee1e6a88ff4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfRevenueAbstract_8a93bd29-95a3-4dcd-9469-65091f22962a" 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_8f5b7ebe-398c-4617-9dab-28b390741ee0" xlink:to="loc_us-gaap_CostOfRevenueAbstract_8a93bd29-95a3-4dcd-9469-65091f22962a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfRevenue_45dc65ef-a1d1-43b1-b5f4-3809b32387ec" 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_8a93bd29-95a3-4dcd-9469-65091f22962a" xlink:to="loc_us-gaap_CostOfRevenue_45dc65ef-a1d1-43b1-b5f4-3809b32387ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GrossProfit_fbbcf6b5-519e-4712-ad32-6b727bc3ba23" 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_8f5b7ebe-398c-4617-9dab-28b390741ee0" xlink:to="loc_us-gaap_GrossProfit_fbbcf6b5-519e-4712-ad32-6b727bc3ba23" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostsAndExpensesAbstract_a543d22d-81a2-4007-a1c3-1fb9344e3b68" 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_8f5b7ebe-398c-4617-9dab-28b390741ee0" xlink:to="loc_us-gaap_CostsAndExpensesAbstract_a543d22d-81a2-4007-a1c3-1fb9344e3b68" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_OperationsProductAndTechnologyExpense_ab485260-5230-485a-ab22-1e2f0a775590" xlink:href="tdup-20210930.xsd#tdup_OperationsProductAndTechnologyExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_a543d22d-81a2-4007-a1c3-1fb9344e3b68" xlink:to="loc_tdup_OperationsProductAndTechnologyExpense_ab485260-5230-485a-ab22-1e2f0a775590" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketingExpense_da19d742-cfd5-499a-8f6a-d3fe80f74447" 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_a543d22d-81a2-4007-a1c3-1fb9344e3b68" xlink:to="loc_us-gaap_MarketingExpense_da19d742-cfd5-499a-8f6a-d3fe80f74447" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpense_faa0bc41-cffd-4269-83ed-69494feab487" 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_a543d22d-81a2-4007-a1c3-1fb9344e3b68" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpense_faa0bc41-cffd-4269-83ed-69494feab487" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpenses_381cb0da-0e3a-45b0-a941-a49f3eef9140" 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_a543d22d-81a2-4007-a1c3-1fb9344e3b68" xlink:to="loc_us-gaap_OperatingExpenses_381cb0da-0e3a-45b0-a941-a49f3eef9140" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_46a3fce0-d93a-4e94-a88b-e8452a75c401" 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_8f5b7ebe-398c-4617-9dab-28b390741ee0" xlink:to="loc_us-gaap_OperatingIncomeLoss_46a3fce0-d93a-4e94-a88b-e8452a75c401" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_58a16969-b355-4217-a21f-44445cd05d39" 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_8f5b7ebe-398c-4617-9dab-28b390741ee0" xlink:to="loc_us-gaap_InterestExpense_58a16969-b355-4217-a21f-44445cd05d39" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_910ecaca-bbaa-4fc7-b163-753d870076f6" 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_8f5b7ebe-398c-4617-9dab-28b390741ee0" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_910ecaca-bbaa-4fc7-b163-753d870076f6" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_8d4c786c-7b81-471d-b6e0-107bc96f999d" 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_8f5b7ebe-398c-4617-9dab-28b390741ee0" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_8d4c786c-7b81-471d-b6e0-107bc96f999d" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_9a5ebff0-9457-4ecf-9f2c-d7b10656b8f3" 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_8f5b7ebe-398c-4617-9dab-28b390741ee0" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_9a5ebff0-9457-4ecf-9f2c-d7b10656b8f3" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_b15b477d-ddce-4568-a089-3bece5c97f9c" 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_8f5b7ebe-398c-4617-9dab-28b390741ee0" xlink:to="loc_us-gaap_NetIncomeLoss_b15b477d-ddce-4568-a089-3bece5c97f9c" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_65876102-3d57-4db9-ac31-fc331da7603e" 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_8f5b7ebe-398c-4617-9dab-28b390741ee0" xlink:to="loc_us-gaap_EarningsPerShareDiluted_65876102-3d57-4db9-ac31-fc331da7603e" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_f3b57ae3-4aad-4f6d-9336-cb08b0ce51df" 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_8f5b7ebe-398c-4617-9dab-28b390741ee0" xlink:to="loc_us-gaap_EarningsPerShareBasic_f3b57ae3-4aad-4f6d-9336-cb08b0ce51df" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_1be7b9cb-f2c1-495c-8b62-23f319b2156a" 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_8f5b7ebe-398c-4617-9dab-28b390741ee0" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_1be7b9cb-f2c1-495c-8b62-23f319b2156a" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_7af82067-e6ce-49f6-9488-bf3ecceb007a" 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_8f5b7ebe-398c-4617-9dab-28b390741ee0" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_7af82067-e6ce-49f6-9488-bf3ecceb007a" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_047a9f16-e491-490e-b19f-3bf932cc78d9" 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_8f5b7ebe-398c-4617-9dab-28b390741ee0" xlink:to="loc_us-gaap_StatementTable_047a9f16-e491-490e-b19f-3bf932cc78d9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_42a2b45a-4277-48c8-ad14-a05411d3b225" 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_047a9f16-e491-490e-b19f-3bf932cc78d9" xlink:to="loc_srt_ProductOrServiceAxis_42a2b45a-4277-48c8-ad14-a05411d3b225" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_42a2b45a-4277-48c8-ad14-a05411d3b225_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_42a2b45a-4277-48c8-ad14-a05411d3b225" xlink:to="loc_srt_ProductsAndServicesDomain_42a2b45a-4277-48c8-ad14-a05411d3b225_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_905efc88-6e2a-4bda-97ae-539bd52d5235" 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_42a2b45a-4277-48c8-ad14-a05411d3b225" xlink:to="loc_srt_ProductsAndServicesDomain_905efc88-6e2a-4bda-97ae-539bd52d5235" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ConsignmentMember_61afa17a-d81b-4fc3-b53a-1b7ce2a412be" xlink:href="tdup-20210930.xsd#tdup_ConsignmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_905efc88-6e2a-4bda-97ae-539bd52d5235" xlink:to="loc_tdup_ConsignmentMember_61afa17a-d81b-4fc3-b53a-1b7ce2a412be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductMember_2fc09674-a475-4f8b-8869-75ad0a5899d9" 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_905efc88-6e2a-4bda-97ae-539bd52d5235" xlink:to="loc_us-gaap_ProductMember_2fc09674-a475-4f8b-8869-75ad0a5899d9" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/CondensedConsolidatedStatementsofConvertiblePreferredStockandStockholdersEquity" xlink:type="simple" xlink:href="tdup-20210930.xsd#CondensedConsolidatedStatementsofConvertiblePreferredStockandStockholdersEquity"/>
  <link:definitionLink xlink:role="http://thredup.com/role/CondensedConsolidatedStatementsofConvertiblePreferredStockandStockholdersEquity" xlink:type="extended" id="iff23a3db1ace494ca47995a4046e77f9_CondensedConsolidatedStatementsofConvertiblePreferredStockandStockholdersEquity">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_dd8cfeb0-b418-4750-b10e-f8999db84bc8" 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_1bc6abc4-5876-43af-89d8-b04fffede4a0" 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_dd8cfeb0-b418-4750-b10e-f8999db84bc8" xlink:to="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_1bc6abc4-5876-43af-89d8-b04fffede4a0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesOutstanding_8f6310ec-22c1-4a46-80b0-0687a2d7a0ef" 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_1bc6abc4-5876-43af-89d8-b04fffede4a0" xlink:to="loc_us-gaap_TemporaryEquitySharesOutstanding_8f6310ec-22c1-4a46-80b0-0687a2d7a0ef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_d3067f5e-81bb-47a4-a073-ba9dc4cd3858" 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_1bc6abc4-5876-43af-89d8-b04fffede4a0" xlink:to="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_d3067f5e-81bb-47a4-a073-ba9dc4cd3858" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_TemporaryEquityConversionOfConvertibleSecuritiesShares_eb30e670-c2fe-43a1-84e4-64b90df849e8" xlink:href="tdup-20210930.xsd#tdup_TemporaryEquityConversionOfConvertibleSecuritiesShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_1bc6abc4-5876-43af-89d8-b04fffede4a0" xlink:to="loc_tdup_TemporaryEquityConversionOfConvertibleSecuritiesShares_eb30e670-c2fe-43a1-84e4-64b90df849e8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_TemporaryEquityConversionOfConvertibleSecuritiesValue_a5b48e92-e86a-4f0a-b8db-8ad0f10252ab" xlink:href="tdup-20210930.xsd#tdup_TemporaryEquityConversionOfConvertibleSecuritiesValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_1bc6abc4-5876-43af-89d8-b04fffede4a0" xlink:to="loc_tdup_TemporaryEquityConversionOfConvertibleSecuritiesValue_a5b48e92-e86a-4f0a-b8db-8ad0f10252ab" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_TemporaryEquityStockIssuedDuringPeriodSharesNewIssues_cd2dbfe9-b869-4fd3-ab9a-cf2e3763169a" xlink:href="tdup-20210930.xsd#tdup_TemporaryEquityStockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_1bc6abc4-5876-43af-89d8-b04fffede4a0" xlink:to="loc_tdup_TemporaryEquityStockIssuedDuringPeriodSharesNewIssues_cd2dbfe9-b869-4fd3-ab9a-cf2e3763169a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityStockIssuedDuringPeriodValueNewIssues_6d1911eb-1b6f-47e4-85c8-0a49c91ba1fb" 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_1bc6abc4-5876-43af-89d8-b04fffede4a0" xlink:to="loc_us-gaap_TemporaryEquityStockIssuedDuringPeriodValueNewIssues_6d1911eb-1b6f-47e4-85c8-0a49c91ba1fb" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesOutstanding_b855c5c5-c606-4cbc-a766-53b6e2745ca8" 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_c57ea44a-6dac-49c9-b3a8-aedc14131594" 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_fdb7857b-a3a6-42c6-ace9-cba4c3f6c99a" 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_dd8cfeb0-b418-4750-b10e-f8999db84bc8" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fdb7857b-a3a6-42c6-ace9-cba4c3f6c99a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_6f79b9a5-bb5a-4021-a202-8b44211bf92a" 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_fdb7857b-a3a6-42c6-ace9-cba4c3f6c99a" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_6f79b9a5-bb5a-4021-a202-8b44211bf92a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_670b6db2-a052-4cf1-9b8a-a126add5cdbf" 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_fdb7857b-a3a6-42c6-ace9-cba4c3f6c99a" xlink:to="loc_us-gaap_StockholdersEquity_670b6db2-a052-4cf1-9b8a-a126add5cdbf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_fc3c3a79-9a1a-40fc-850e-ec7a36f3269e" 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_fdb7857b-a3a6-42c6-ace9-cba4c3f6c99a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_fc3c3a79-9a1a-40fc-850e-ec7a36f3269e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_f04f1964-586e-4fd6-ab14-c081c14743c4" 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_fdb7857b-a3a6-42c6-ace9-cba4c3f6c99a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_f04f1964-586e-4fd6-ab14-c081c14743c4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_e5fed8a1-288a-416f-87cb-98566d7e2415" 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_fdb7857b-a3a6-42c6-ace9-cba4c3f6c99a" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_e5fed8a1-288a-416f-87cb-98566d7e2415" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_AdjustmentsToAdditionalPaidInCapitalConversionOfConvertibleWarrants_77307a4b-14e7-4400-b622-90147f66b5d3" xlink:href="tdup-20210930.xsd#tdup_AdjustmentsToAdditionalPaidInCapitalConversionOfConvertibleWarrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fdb7857b-a3a6-42c6-ace9-cba4c3f6c99a" xlink:to="loc_tdup_AdjustmentsToAdditionalPaidInCapitalConversionOfConvertibleWarrants_77307a4b-14e7-4400-b622-90147f66b5d3" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_2e59acbe-0587-4648-9d7b-163ff77d5664" 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_fdb7857b-a3a6-42c6-ace9-cba4c3f6c99a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_2e59acbe-0587-4648-9d7b-163ff77d5664" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities_b77f137d-4da4-4513-8218-3b15b260cbfb" 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_fdb7857b-a3a6-42c6-ace9-cba4c3f6c99a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities_b77f137d-4da4-4513-8218-3b15b260cbfb" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_014ffd91-b5ee-4531-b24b-babfc754a938" 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_fdb7857b-a3a6-42c6-ace9-cba4c3f6c99a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_014ffd91-b5ee-4531-b24b-babfc754a938" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_073d837b-a94c-4235-8292-0b9ae1092573" 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_fdb7857b-a3a6-42c6-ace9-cba4c3f6c99a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_073d837b-a94c-4235-8292-0b9ae1092573" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_StockIssuedDuringPeriodSharesWarrantsIssued_cfb714d8-c8d2-4dee-a2ac-411c97012bdf" xlink:href="tdup-20210930.xsd#tdup_StockIssuedDuringPeriodSharesWarrantsIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_fdb7857b-a3a6-42c6-ace9-cba4c3f6c99a" xlink:to="loc_tdup_StockIssuedDuringPeriodSharesWarrantsIssued_cfb714d8-c8d2-4dee-a2ac-411c97012bdf" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_67b71c7f-1400-4ed6-9cf2-b3d9c736d542" 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_fdb7857b-a3a6-42c6-ace9-cba4c3f6c99a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_67b71c7f-1400-4ed6-9cf2-b3d9c736d542" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_792d6092-830e-4fec-82dc-a430b7f82606" 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_fdb7857b-a3a6-42c6-ace9-cba4c3f6c99a" xlink:to="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_792d6092-830e-4fec-82dc-a430b7f82606" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_649f9ce8-1450-46e4-a2ad-036ed1236ea2" 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_fdb7857b-a3a6-42c6-ace9-cba4c3f6c99a" xlink:to="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_649f9ce8-1450-46e4-a2ad-036ed1236ea2" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_40d65f70-4500-4254-b584-fa9bc35aee5f" 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_fdb7857b-a3a6-42c6-ace9-cba4c3f6c99a" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_40d65f70-4500-4254-b584-fa9bc35aee5f" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_014b2e6e-576d-4e4f-a14e-5dead6e99cb7" 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_fdb7857b-a3a6-42c6-ace9-cba4c3f6c99a" xlink:to="loc_us-gaap_NetIncomeLoss_014b2e6e-576d-4e4f-a14e-5dead6e99cb7" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_1ae568a1-fdd7-42de-8c55-c4b54b3192ff" 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_7b37fa77-178d-4105-a274-66a3880b9a6a" 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_d4ccca0b-e117-4a19-9323-969f1a9408f6" 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_dd8cfeb0-b418-4750-b10e-f8999db84bc8" xlink:to="loc_us-gaap_AccountingStandardsUpdateExtensibleList_d4ccca0b-e117-4a19-9323-969f1a9408f6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_51ae51b2-8077-4d16-8a5f-8de3624630c1" 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_dd8cfeb0-b418-4750-b10e-f8999db84bc8" xlink:to="loc_us-gaap_StatementTable_51ae51b2-8077-4d16-8a5f-8de3624630c1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_7102d31a-107f-4c22-8dad-2f59905d43e0" 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_51ae51b2-8077-4d16-8a5f-8de3624630c1" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_7102d31a-107f-4c22-8dad-2f59905d43e0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_7102d31a-107f-4c22-8dad-2f59905d43e0_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_7102d31a-107f-4c22-8dad-2f59905d43e0" xlink:to="loc_us-gaap_EquityComponentDomain_7102d31a-107f-4c22-8dad-2f59905d43e0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_819721c8-1a84-4659-853f-64fb0cc54890" 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_7102d31a-107f-4c22-8dad-2f59905d43e0" xlink:to="loc_us-gaap_EquityComponentDomain_819721c8-1a84-4659-853f-64fb0cc54890" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_27892a4f-d835-4769-a948-00221b311ca6" 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_819721c8-1a84-4659-853f-64fb0cc54890" xlink:to="loc_us-gaap_CommonStockMember_27892a4f-d835-4769-a948-00221b311ca6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_c561d127-41d3-4711-82b8-b7d818fe2df0" 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_819721c8-1a84-4659-853f-64fb0cc54890" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_c561d127-41d3-4711-82b8-b7d818fe2df0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_b4d348f5-bac0-463b-882d-8108ed37a14f" 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_819721c8-1a84-4659-853f-64fb0cc54890" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_b4d348f5-bac0-463b-882d-8108ed37a14f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_298fdb6a-7ee5-469e-8a83-fbba0939d423" 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_819721c8-1a84-4659-853f-64fb0cc54890" xlink:to="loc_us-gaap_RetainedEarningsMember_298fdb6a-7ee5-469e-8a83-fbba0939d423" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_7e5a8f70-eab6-4c22-a4d9-65eda3c9182f" 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_51ae51b2-8077-4d16-8a5f-8de3624630c1" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_7e5a8f70-eab6-4c22-a4d9-65eda3c9182f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_7e5a8f70-eab6-4c22-a4d9-65eda3c9182f_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_7e5a8f70-eab6-4c22-a4d9-65eda3c9182f" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_7e5a8f70-eab6-4c22-a4d9-65eda3c9182f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_c5600f3b-ed0a-4592-ad3b-14694a4dbee0" 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_7e5a8f70-eab6-4c22-a4d9-65eda3c9182f" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_c5600f3b-ed0a-4592-ad3b-14694a4dbee0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_dce3408f-c18a-4cee-9017-a2cfc7ee1664" 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_c5600f3b-ed0a-4592-ad3b-14694a4dbee0" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_dce3408f-c18a-4cee-9017-a2cfc7ee1664" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/OrganizationandDescriptionofBusinessDetails" xlink:type="simple" xlink:href="tdup-20210930.xsd#OrganizationandDescriptionofBusinessDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/OrganizationandDescriptionofBusinessDetails" xlink:type="extended" id="id9e973ed1ee84cfcabbe1dd5c8d0f947_OrganizationandDescriptionofBusinessDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_4fc48e1a-8ded-440c-96d3-3982e68d8e8b" 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_c29ae410-f3f4-42fd-ba09-9596aa255dab" 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_4fc48e1a-8ded-440c-96d3-3982e68d8e8b" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_c29ae410-f3f4-42fd-ba09-9596aa255dab" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_60f136ad-4160-456e-8daf-cde11e688cfa" 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_4fc48e1a-8ded-440c-96d3-3982e68d8e8b" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_60f136ad-4160-456e-8daf-cde11e688cfa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_d59a8a28-7d06-403f-b61b-d7ed4a57ae7d" 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_4fc48e1a-8ded-440c-96d3-3982e68d8e8b" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_d59a8a28-7d06-403f-b61b-d7ed4a57ae7d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts_ba259d27-da52-4a12-be47-f80fcea25d58" 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_4fc48e1a-8ded-440c-96d3-3982e68d8e8b" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts_ba259d27-da52-4a12-be47-f80fcea25d58" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_612b76c2-ea1b-4bed-914b-6f8f697d778b" 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_4fc48e1a-8ded-440c-96d3-3982e68d8e8b" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_612b76c2-ea1b-4bed-914b-6f8f697d778b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesAuthorized_68293e61-0b33-4dea-b15d-aa54fca7a40c" 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_4fc48e1a-8ded-440c-96d3-3982e68d8e8b" xlink:to="loc_us-gaap_TemporaryEquitySharesAuthorized_68293e61-0b33-4dea-b15d-aa54fca7a40c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_cfd8318a-96ca-4f54-bcb0-81b9b8e9fead" 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_4fc48e1a-8ded-440c-96d3-3982e68d8e8b" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_cfd8318a-96ca-4f54-bcb0-81b9b8e9fead" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ReclassificationOfCommonStockBetweenClassesShares_d89a05a8-ee66-4e49-a15c-14d72bfaf9e8" xlink:href="tdup-20210930.xsd#tdup_ReclassificationOfCommonStockBetweenClassesShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_4fc48e1a-8ded-440c-96d3-3982e68d8e8b" xlink:to="loc_tdup_ReclassificationOfCommonStockBetweenClassesShares_d89a05a8-ee66-4e49-a15c-14d72bfaf9e8" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_71b9c575-304a-4349-900b-ed74b75da0f0" 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_4fc48e1a-8ded-440c-96d3-3982e68d8e8b" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_71b9c575-304a-4349-900b-ed74b75da0f0" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfStockIssuanceCosts_da98409b-280f-41bc-b073-cc468706c333" 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_4fc48e1a-8ded-440c-96d3-3982e68d8e8b" xlink:to="loc_us-gaap_PaymentsOfStockIssuanceCosts_da98409b-280f-41bc-b073-cc468706c333" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_a98a166a-2d91-47e3-bd68-0259fe5db2ac" 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_ClassOfStockLineItems_4fc48e1a-8ded-440c-96d3-3982e68d8e8b" xlink:to="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_a98a166a-2d91-47e3-bd68-0259fe5db2ac" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_e5d7818c-3b73-4d1f-b247-9397469c78f2" 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_ClassOfStockLineItems_4fc48e1a-8ded-440c-96d3-3982e68d8e8b" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_e5d7818c-3b73-4d1f-b247-9397469c78f2" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfAssumedDebt_18a7f22d-e276-4baa-8f32-0c49bd20b5cf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RepaymentsOfAssumedDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_4fc48e1a-8ded-440c-96d3-3982e68d8e8b" xlink:to="loc_us-gaap_RepaymentsOfAssumedDebt_18a7f22d-e276-4baa-8f32-0c49bd20b5cf" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_a01a9c39-c634-4f61-8ecb-99f365993b2a" 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_ClassOfStockLineItems_4fc48e1a-8ded-440c-96d3-3982e68d8e8b" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_a01a9c39-c634-4f61-8ecb-99f365993b2a" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_dbe524ca-84e7-4f55-9740-ec03684d238d" 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_ClassOfStockLineItems_4fc48e1a-8ded-440c-96d3-3982e68d8e8b" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable_dbe524ca-84e7-4f55-9740-ec03684d238d" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued_8d561ebf-ff34-4ecd-9e71-8b7e754176cd" 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_ClassOfStockLineItems_4fc48e1a-8ded-440c-96d3-3982e68d8e8b" xlink:to="loc_us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued_8d561ebf-ff34-4ecd-9e71-8b7e754176cd" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_BusinessAcquisitionEquityInterestIssuedOrIssuableNumberOfSharesIssuancePeriod_a7c23493-5800-4f53-adfa-dd1452826243" xlink:href="tdup-20210930.xsd#tdup_BusinessAcquisitionEquityInterestIssuedOrIssuableNumberOfSharesIssuancePeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_4fc48e1a-8ded-440c-96d3-3982e68d8e8b" xlink:to="loc_tdup_BusinessAcquisitionEquityInterestIssuedOrIssuableNumberOfSharesIssuancePeriod_a7c23493-5800-4f53-adfa-dd1452826243" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_7893e5be-31d7-44b4-a55d-864b2caf24ee" 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_4fc48e1a-8ded-440c-96d3-3982e68d8e8b" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_7893e5be-31d7-44b4-a55d-864b2caf24ee" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_b304db0d-b30c-4a39-aff0-bedb04990083" 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_7893e5be-31d7-44b4-a55d-864b2caf24ee" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_b304db0d-b30c-4a39-aff0-bedb04990083" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_b304db0d-b30c-4a39-aff0-bedb04990083_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_b304db0d-b30c-4a39-aff0-bedb04990083" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_b304db0d-b30c-4a39-aff0-bedb04990083_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_90eecd4f-9194-476e-a4e6-1cb7a9db9db5" 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_b304db0d-b30c-4a39-aff0-bedb04990083" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_90eecd4f-9194-476e-a4e6-1cb7a9db9db5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IPOMember_ef3e0bd7-c872-4c1d-81ab-98b5bafaf766" 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_90eecd4f-9194-476e-a4e6-1cb7a9db9db5" xlink:to="loc_us-gaap_IPOMember_ef3e0bd7-c872-4c1d-81ab-98b5bafaf766" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_e09e1186-3869-4d12-890a-d3a8e44e5f0d" 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_7893e5be-31d7-44b4-a55d-864b2caf24ee" xlink:to="loc_us-gaap_StatementClassOfStockAxis_e09e1186-3869-4d12-890a-d3a8e44e5f0d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_e09e1186-3869-4d12-890a-d3a8e44e5f0d_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_e09e1186-3869-4d12-890a-d3a8e44e5f0d" xlink:to="loc_us-gaap_ClassOfStockDomain_e09e1186-3869-4d12-890a-d3a8e44e5f0d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_18f0d3a5-a772-495e-aeb3-8ca783f8750f" 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_e09e1186-3869-4d12-890a-d3a8e44e5f0d" xlink:to="loc_us-gaap_ClassOfStockDomain_18f0d3a5-a772-495e-aeb3-8ca783f8750f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_856916e7-ec5e-4697-9c74-28660d64969f" 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_18f0d3a5-a772-495e-aeb3-8ca783f8750f" xlink:to="loc_us-gaap_CommonClassAMember_856916e7-ec5e-4697-9c74-28660d64969f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_37f08d1b-8846-439d-81aa-f05695535b4d" 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_18f0d3a5-a772-495e-aeb3-8ca783f8750f" xlink:to="loc_us-gaap_CommonClassBMember_37f08d1b-8846-439d-81aa-f05695535b4d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_6fddf242-6783-408c-99ec-36aa48bcb4f0" 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_7893e5be-31d7-44b4-a55d-864b2caf24ee" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_6fddf242-6783-408c-99ec-36aa48bcb4f0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_6fddf242-6783-408c-99ec-36aa48bcb4f0_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_6fddf242-6783-408c-99ec-36aa48bcb4f0" xlink:to="loc_us-gaap_EquityComponentDomain_6fddf242-6783-408c-99ec-36aa48bcb4f0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_0467a98a-6f94-43bd-9de4-8bf7cb99d5d2" 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_6fddf242-6783-408c-99ec-36aa48bcb4f0" xlink:to="loc_us-gaap_EquityComponentDomain_0467a98a-6f94-43bd-9de4-8bf7cb99d5d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_e8bad9ca-9cc1-494d-9bf8-f17bf0638c08" 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_0467a98a-6f94-43bd-9de4-8bf7cb99d5d2" xlink:to="loc_us-gaap_CommonStockMember_e8bad9ca-9cc1-494d-9bf8-f17bf0638c08" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_5c4a2f6b-2a9e-4dbb-a80b-8d31a2e076e3" 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_7893e5be-31d7-44b4-a55d-864b2caf24ee" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_5c4a2f6b-2a9e-4dbb-a80b-8d31a2e076e3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_5c4a2f6b-2a9e-4dbb-a80b-8d31a2e076e3_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_5c4a2f6b-2a9e-4dbb-a80b-8d31a2e076e3" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_5c4a2f6b-2a9e-4dbb-a80b-8d31a2e076e3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_feecfe1b-85c0-4fb5-89c1-4181b2cee042" 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_5c4a2f6b-2a9e-4dbb-a80b-8d31a2e076e3" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_feecfe1b-85c0-4fb5-89c1-4181b2cee042" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ClassBCommonStockWarrantMember_1d23406f-5897-4b93-9272-c88881253703" xlink:href="tdup-20210930.xsd#tdup_ClassBCommonStockWarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_feecfe1b-85c0-4fb5-89c1-4181b2cee042" xlink:to="loc_tdup_ClassBCommonStockWarrantMember_1d23406f-5897-4b93-9272-c88881253703" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_ac6ca0b9-3ede-4a5f-be1a-61059d4dd5ed" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_7893e5be-31d7-44b4-a55d-864b2caf24ee" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_ac6ca0b9-3ede-4a5f-be1a-61059d4dd5ed" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_ac6ca0b9-3ede-4a5f-be1a-61059d4dd5ed_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_ac6ca0b9-3ede-4a5f-be1a-61059d4dd5ed" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_ac6ca0b9-3ede-4a5f-be1a-61059d4dd5ed_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_c02ee0b2-d737-43a5-8689-ae51a45564a0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_ac6ca0b9-3ede-4a5f-be1a-61059d4dd5ed" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_c02ee0b2-d737-43a5-8689-ae51a45564a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_f3430b02-3028-4e71-a025-bee19fc6a150" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_c02ee0b2-d737-43a5-8689-ae51a45564a0" xlink:to="loc_us-gaap_SubsequentEventMember_f3430b02-3028-4e71-a025-bee19fc6a150" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_9e60f396-453b-464c-b2fb-3f4f71439f7e" 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_ScheduleOfStockByClassTable_7893e5be-31d7-44b4-a55d-864b2caf24ee" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_9e60f396-453b-464c-b2fb-3f4f71439f7e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9e60f396-453b-464c-b2fb-3f4f71439f7e_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_9e60f396-453b-464c-b2fb-3f4f71439f7e" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9e60f396-453b-464c-b2fb-3f4f71439f7e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_fbd4a35f-f79c-4356-959b-f77cc46bfeac" 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_9e60f396-453b-464c-b2fb-3f4f71439f7e" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_fbd4a35f-f79c-4356-959b-f77cc46bfeac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_RemixGlobalADMember_ba3ba9d6-bce9-4a61-89ab-a648dc9f3f84" xlink:href="tdup-20210930.xsd#tdup_RemixGlobalADMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_fbd4a35f-f79c-4356-959b-f77cc46bfeac" xlink:to="loc_tdup_RemixGlobalADMember_ba3ba9d6-bce9-4a61-89ab-a648dc9f3f84" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_e6554597-fa46-4a00-ac66-d8ac2c37e2d7" 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_7893e5be-31d7-44b4-a55d-864b2caf24ee" xlink:to="loc_us-gaap_AwardTypeAxis_e6554597-fa46-4a00-ac66-d8ac2c37e2d7" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e6554597-fa46-4a00-ac66-d8ac2c37e2d7_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_e6554597-fa46-4a00-ac66-d8ac2c37e2d7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e6554597-fa46-4a00-ac66-d8ac2c37e2d7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1e76b42c-4fa3-4987-8111-fd389f8dc9dc" 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_e6554597-fa46-4a00-ac66-d8ac2c37e2d7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1e76b42c-4fa3-4987-8111-fd389f8dc9dc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_76d2bfcd-080d-4522-90fa-29a825fb3933" 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_1e76b42c-4fa3-4987-8111-fd389f8dc9dc" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_76d2bfcd-080d-4522-90fa-29a825fb3933" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/SignificantAccountingPoliciesNarrativeDetails" xlink:type="simple" xlink:href="tdup-20210930.xsd#SignificantAccountingPoliciesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/SignificantAccountingPoliciesNarrativeDetails" xlink:type="extended" id="i1d3deff65bb647ee87c617514fbaf0da_SignificantAccountingPoliciesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_d4648e0c-3f63-4b74-a096-2adb3494aee9" 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_ContractWithCustomerLiabilityRevenueRecognized_83ccc969-807a-470c-979b-724944200c7f" 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_d4648e0c-3f63-4b74-a096-2adb3494aee9" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_83ccc969-807a-470c-979b-724944200c7f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashNoncurrent_52eee974-6829-4ddc-b0f6-30f7feb4b9a4" 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_d4648e0c-3f63-4b74-a096-2adb3494aee9" xlink:to="loc_us-gaap_RestrictedCashNoncurrent_52eee974-6829-4ddc-b0f6-30f7feb4b9a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_3bbd0d0f-7b18-486a-9e74-23e548ff635c" 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_d4648e0c-3f63-4b74-a096-2adb3494aee9" xlink:to="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_3bbd0d0f-7b18-486a-9e74-23e548ff635c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_e3cb9ef0-33da-4e53-ae3d-77a1b8fe2fba" 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_3bbd0d0f-7b18-486a-9e74-23e548ff635c" xlink:to="loc_srt_ProductOrServiceAxis_e3cb9ef0-33da-4e53-ae3d-77a1b8fe2fba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_e3cb9ef0-33da-4e53-ae3d-77a1b8fe2fba_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_e3cb9ef0-33da-4e53-ae3d-77a1b8fe2fba" xlink:to="loc_srt_ProductsAndServicesDomain_e3cb9ef0-33da-4e53-ae3d-77a1b8fe2fba_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_097126b1-35f6-440a-975e-832427d2ef11" 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_e3cb9ef0-33da-4e53-ae3d-77a1b8fe2fba" xlink:to="loc_srt_ProductsAndServicesDomain_097126b1-35f6-440a-975e-832427d2ef11" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_LoyaltyProgramMember_89899fe4-5382-48ed-9c50-1d383995ec4d" xlink:href="tdup-20210930.xsd#tdup_LoyaltyProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_097126b1-35f6-440a-975e-832427d2ef11" xlink:to="loc_tdup_LoyaltyProgramMember_89899fe4-5382-48ed-9c50-1d383995ec4d" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/FinancialInstrumentsandFairValueMeasurementsDetails" xlink:type="simple" xlink:href="tdup-20210930.xsd#FinancialInstrumentsandFairValueMeasurementsDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/FinancialInstrumentsandFairValueMeasurementsDetails" xlink:type="extended" id="ibdafc3a5e56342d299dc2830a33f06c2_FinancialInstrumentsandFairValueMeasurementsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01958f16-fdc1-45ae-94ed-8ed95b801411" 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_76ac51d2-012a-45f3-a25c-cba9aae890c3" 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_01958f16-fdc1-45ae-94ed-8ed95b801411" xlink:to="loc_us-gaap_AssetsFairValueDisclosureAbstract_76ac51d2-012a-45f3-a25c-cba9aae890c3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_9815fa8b-eb06-409d-ab31-0598cbf8c515" 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_76ac51d2-012a-45f3-a25c-cba9aae890c3" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_9815fa8b-eb06-409d-ab31-0598cbf8c515" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_96f68b53-12d0-4ded-846e-831aa03a1f03" 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_76ac51d2-012a-45f3-a25c-cba9aae890c3" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_96f68b53-12d0-4ded-846e-831aa03a1f03" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_eab6ab37-92e2-45ec-9c2f-79f040ad41ee" 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_76ac51d2-012a-45f3-a25c-cba9aae890c3" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_eab6ab37-92e2-45ec-9c2f-79f040ad41ee" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAbstract_76f9b05d-910c-4284-9cc5-3db78595a6e5" 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_01958f16-fdc1-45ae-94ed-8ed95b801411" xlink:to="loc_us-gaap_CashAndCashEquivalentsAbstract_76f9b05d-910c-4284-9cc5-3db78595a6e5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_f7c57d89-6e0d-4a39-9083-092c47a11239" 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_76f9b05d-910c-4284-9cc5-3db78595a6e5" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_f7c57d89-6e0d-4a39-9083-092c47a11239" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstanding_5fd30889-720f-4edb-ae90-a9af386dcc33" 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_01958f16-fdc1-45ae-94ed-8ed95b801411" xlink:to="loc_us-gaap_WarrantsAndRightsOutstanding_5fd30889-720f-4edb-ae90-a9af386dcc33" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue_d4d902d6-ee65-4ec7-b373-7a9fa2dcda03" 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_01958f16-fdc1-45ae-94ed-8ed95b801411" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue_d4d902d6-ee65-4ec7-b373-7a9fa2dcda03" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue_8eb17b84-fc81-41da-b8bb-ea0b5476c4c3" 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_01958f16-fdc1-45ae-94ed-8ed95b801411" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue_8eb17b84-fc81-41da-b8bb-ea0b5476c4c3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_f8ecd2a0-8f32-4d3e-ba2a-2350bd6a08db" 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_01958f16-fdc1-45ae-94ed-8ed95b801411" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_f8ecd2a0-8f32-4d3e-ba2a-2350bd6a08db" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_157d41ff-de6f-4e84-9fd4-59a49ac1c0d0" 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_f8ecd2a0-8f32-4d3e-ba2a-2350bd6a08db" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_157d41ff-de6f-4e84-9fd4-59a49ac1c0d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_157d41ff-de6f-4e84-9fd4-59a49ac1c0d0_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_157d41ff-de6f-4e84-9fd4-59a49ac1c0d0" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_157d41ff-de6f-4e84-9fd4-59a49ac1c0d0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_26082260-5a27-4ac8-a431-5a52231c530d" 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_157d41ff-de6f-4e84-9fd4-59a49ac1c0d0" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_26082260-5a27-4ac8-a431-5a52231c530d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_6ed1b708-5ea4-4812-a92f-a5e07dd0b77f" 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_26082260-5a27-4ac8-a431-5a52231c530d" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_6ed1b708-5ea4-4812-a92f-a5e07dd0b77f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_1bd2cbb1-a417-47eb-9146-91d77ca8ea32" 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_f8ecd2a0-8f32-4d3e-ba2a-2350bd6a08db" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_1bd2cbb1-a417-47eb-9146-91d77ca8ea32" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1bd2cbb1-a417-47eb-9146-91d77ca8ea32_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_1bd2cbb1-a417-47eb-9146-91d77ca8ea32" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1bd2cbb1-a417-47eb-9146-91d77ca8ea32_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_177bb25c-7e13-4455-83dc-69e47765a3ff" 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_1bd2cbb1-a417-47eb-9146-91d77ca8ea32" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_177bb25c-7e13-4455-83dc-69e47765a3ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_0730d284-d759-415a-a971-fdd1da625450" 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_177bb25c-7e13-4455-83dc-69e47765a3ff" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_0730d284-d759-415a-a971-fdd1da625450" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_4715d7f8-76fe-4e45-97b2-9bd69cbdf858" 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_177bb25c-7e13-4455-83dc-69e47765a3ff" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_4715d7f8-76fe-4e45-97b2-9bd69cbdf858" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_11c6da25-2242-46d4-a85a-d94702faf695" 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_177bb25c-7e13-4455-83dc-69e47765a3ff" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_11c6da25-2242-46d4-a85a-d94702faf695" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_7c43dab0-723e-4e4d-81cd-5648fe7c303f" 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_f8ecd2a0-8f32-4d3e-ba2a-2350bd6a08db" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_7c43dab0-723e-4e4d-81cd-5648fe7c303f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_7c43dab0-723e-4e4d-81cd-5648fe7c303f_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_7c43dab0-723e-4e4d-81cd-5648fe7c303f" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_7c43dab0-723e-4e4d-81cd-5648fe7c303f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_ee038c14-3dce-4503-b108-9a62e610cd67" 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_7c43dab0-723e-4e4d-81cd-5648fe7c303f" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_ee038c14-3dce-4503-b108-9a62e610cd67" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_91dd0d48-1db0-4027-8c4c-49e4b934e0ac" 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_ee038c14-3dce-4503-b108-9a62e610cd67" xlink:to="loc_us-gaap_MoneyMarketFundsMember_91dd0d48-1db0-4027-8c4c-49e4b934e0ac" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_6ad2e2a8-bf43-424f-9365-a3220eb7c18c" 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_ee038c14-3dce-4503-b108-9a62e610cd67" xlink:to="loc_us-gaap_CommercialPaperMember_6ad2e2a8-bf43-424f-9365-a3220eb7c18c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_76cf39f9-2694-44e1-8968-3a98f7536db5" 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_f8ecd2a0-8f32-4d3e-ba2a-2350bd6a08db" xlink:to="loc_us-gaap_FinancialInstrumentAxis_76cf39f9-2694-44e1-8968-3a98f7536db5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_76cf39f9-2694-44e1-8968-3a98f7536db5_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_76cf39f9-2694-44e1-8968-3a98f7536db5" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_76cf39f9-2694-44e1-8968-3a98f7536db5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_1b7cd6ce-e916-4e6a-bee3-099dd6de884e" 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_76cf39f9-2694-44e1-8968-3a98f7536db5" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_1b7cd6ce-e916-4e6a-bee3-099dd6de884e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_c5948a4a-c931-41c5-a3c4-b2923cfc560a" 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_1b7cd6ce-e916-4e6a-bee3-099dd6de884e" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_c5948a4a-c931-41c5-a3c4-b2923cfc560a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_c1fad55f-f21c-475a-ac67-8cfafc4d6814" 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_1b7cd6ce-e916-4e6a-bee3-099dd6de884e" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_c1fad55f-f21c-475a-ac67-8cfafc4d6814" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_837e7436-8d17-4824-874f-178905ec9722" 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_1b7cd6ce-e916-4e6a-bee3-099dd6de884e" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_837e7436-8d17-4824-874f-178905ec9722" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/LeaseAgreementsNarrativeDetails" xlink:type="simple" xlink:href="tdup-20210930.xsd#LeaseAgreementsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/LeaseAgreementsNarrativeDetails" xlink:type="extended" id="icc4d49899cf44eaab6db53edb871733d_LeaseAgreementsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_4e1be51d-b317-469d-8326-0bcedbee0e63" 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_OperatingLeaseExpense_20097b6c-89b0-468b-b26d-a2dc00df6331" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLeaseExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_4e1be51d-b317-469d-8326-0bcedbee0e63" xlink:to="loc_us-gaap_OperatingLeaseExpense_20097b6c-89b0-468b-b26d-a2dc00df6331" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1_0f9209c7-e252-4c10-9e2a-21269e381b12" 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_4e1be51d-b317-469d-8326-0bcedbee0e63" xlink:to="loc_us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1_0f9209c7-e252-4c10-9e2a-21269e381b12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_LesseeOperatingLeaseLeaseNotYetCommencedExpense_b70a2fd6-ec8b-4f82-b969-c2688268b16b" xlink:href="tdup-20210930.xsd#tdup_LesseeOperatingLeaseLeaseNotYetCommencedExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_4e1be51d-b317-469d-8326-0bcedbee0e63" xlink:to="loc_tdup_LesseeOperatingLeaseLeaseNotYetCommencedExpense_b70a2fd6-ec8b-4f82-b969-c2688268b16b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_LesseeOperatingLeaseLeaseNotYetCommencedRentImprovementAllowance_c34c14f0-2728-403e-aa4c-d71818a175f3" xlink:href="tdup-20210930.xsd#tdup_LesseeOperatingLeaseLeaseNotYetCommencedRentImprovementAllowance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_4e1be51d-b317-469d-8326-0bcedbee0e63" xlink:to="loc_tdup_LesseeOperatingLeaseLeaseNotYetCommencedRentImprovementAllowance_c34c14f0-2728-403e-aa4c-d71818a175f3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_7a6ad177-89f4-4733-bc88-011fb09d68d0" 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_4e1be51d-b317-469d-8326-0bcedbee0e63" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_7a6ad177-89f4-4733-bc88-011fb09d68d0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_6c75b4ce-42b3-4ef3-8ca8-8abb8a51345e" 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_4e1be51d-b317-469d-8326-0bcedbee0e63" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_6c75b4ce-42b3-4ef3-8ca8-8abb8a51345e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_ac4a576a-6a51-4025-bf86-f379ba5ad112" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_6c75b4ce-42b3-4ef3-8ca8-8abb8a51345e" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_ac4a576a-6a51-4025-bf86-f379ba5ad112" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_ac4a576a-6a51-4025-bf86-f379ba5ad112_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_ac4a576a-6a51-4025-bf86-f379ba5ad112" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_ac4a576a-6a51-4025-bf86-f379ba5ad112_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_34f9e191-40d7-4a03-83ff-099fcf8641b5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_ac4a576a-6a51-4025-bf86-f379ba5ad112" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_34f9e191-40d7-4a03-83ff-099fcf8641b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncurrentAssetsMember_416151e4-6ad7-4163-8eb1-8799f19e234a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherNoncurrentAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_34f9e191-40d7-4a03-83ff-099fcf8641b5" xlink:to="loc_us-gaap_OtherNoncurrentAssetsMember_416151e4-6ad7-4163-8eb1-8799f19e234a" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/LongtermDebtandConvertiblePreferredStockWarrantsNarrativeDetails" xlink:type="simple" xlink:href="tdup-20210930.xsd#LongtermDebtandConvertiblePreferredStockWarrantsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/LongtermDebtandConvertiblePreferredStockWarrantsNarrativeDetails" xlink:type="extended" id="i1bda0091ee8646fcb4306d10fd480f28_LongtermDebtandConvertiblePreferredStockWarrantsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_6e83a644-306f-43dc-b08b-7e26851d8c2d" 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_be850618-cd34-457f-88ca-79d4775a7517" xlink:href="tdup-20210930.xsd#tdup_DebtInstrumentMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6e83a644-306f-43dc-b08b-7e26851d8c2d" xlink:to="loc_tdup_DebtInstrumentMaximumBorrowingCapacity_be850618-cd34-457f-88ca-79d4775a7517" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebt_64c35754-37c1-4b65-8c8c-2b3f83fe362c" 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_6e83a644-306f-43dc-b08b-7e26851d8c2d" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebt_64c35754-37c1-4b65-8c8c-2b3f83fe362c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_399a8bc9-0a6a-44b2-8b9e-629fbf66a238" 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_6e83a644-306f-43dc-b08b-7e26851d8c2d" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_399a8bc9-0a6a-44b2-8b9e-629fbf66a238" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_8b7dff41-6036-47c2-88a4-86385d44c86a" 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_6e83a644-306f-43dc-b08b-7e26851d8c2d" xlink:to="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_8b7dff41-6036-47c2-88a4-86385d44c86a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ClassOfWarrantAndRightsCalculationPercentageOfLongTermDebtDraw_eb195cac-7f72-4532-8894-180d63ef35ba" xlink:href="tdup-20210930.xsd#tdup_ClassOfWarrantAndRightsCalculationPercentageOfLongTermDebtDraw"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6e83a644-306f-43dc-b08b-7e26851d8c2d" xlink:to="loc_tdup_ClassOfWarrantAndRightsCalculationPercentageOfLongTermDebtDraw_eb195cac-7f72-4532-8894-180d63ef35ba" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_581153a2-26b1-42a8-bac6-e4b2702c43a6" 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_6e83a644-306f-43dc-b08b-7e26851d8c2d" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_581153a2-26b1-42a8-bac6-e4b2702c43a6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_0e861cfb-b6a3-4763-8624-8b2407cedc64" 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_6e83a644-306f-43dc-b08b-7e26851d8c2d" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_0e861cfb-b6a3-4763-8624-8b2407cedc64" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_b74f6691-06e1-4887-ab0d-f910ccb9cf87" 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_6e83a644-306f-43dc-b08b-7e26851d8c2d" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_b74f6691-06e1-4887-ab0d-f910ccb9cf87" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstanding_dd56003f-5dab-4ae7-93ac-3a4d8e2d569b" 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_6e83a644-306f-43dc-b08b-7e26851d8c2d" xlink:to="loc_us-gaap_WarrantsAndRightsOutstanding_dd56003f-5dab-4ae7-93ac-3a4d8e2d569b" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_c64ecfb0-a112-491c-8166-e1659a29bb6f" 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_6e83a644-306f-43dc-b08b-7e26851d8c2d" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_c64ecfb0-a112-491c-8166-e1659a29bb6f" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_db9eb1c0-3ee0-4f45-9efd-a1a54a873268" 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_6e83a644-306f-43dc-b08b-7e26851d8c2d" xlink:to="loc_us-gaap_DebtInstrumentTable_db9eb1c0-3ee0-4f45-9efd-a1a54a873268" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_c9149ab4-3041-486c-8f06-739eb4047a77" 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_db9eb1c0-3ee0-4f45-9efd-a1a54a873268" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_c9149ab4-3041-486c-8f06-739eb4047a77" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_c9149ab4-3041-486c-8f06-739eb4047a77_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_c9149ab4-3041-486c-8f06-739eb4047a77" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_c9149ab4-3041-486c-8f06-739eb4047a77_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_9dda310b-6677-4451-be5c-2eff194ab1b5" 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_c9149ab4-3041-486c-8f06-739eb4047a77" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_9dda310b-6677-4451-be5c-2eff194ab1b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MediumTermNotesMember_bc3f10ff-70e2-4350-9d5e-cf29c5daba65" 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_9dda310b-6677-4451-be5c-2eff194ab1b5" xlink:to="loc_us-gaap_MediumTermNotesMember_bc3f10ff-70e2-4350-9d5e-cf29c5daba65" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_4f1bc9b9-dbdd-4d42-aca7-e4a47157839c" 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_db9eb1c0-3ee0-4f45-9efd-a1a54a873268" xlink:to="loc_us-gaap_DebtInstrumentAxis_4f1bc9b9-dbdd-4d42-aca7-e4a47157839c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_4f1bc9b9-dbdd-4d42-aca7-e4a47157839c_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_4f1bc9b9-dbdd-4d42-aca7-e4a47157839c" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_4f1bc9b9-dbdd-4d42-aca7-e4a47157839c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_339a81cd-7ca4-47d4-ad9c-5137008799f6" 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_4f1bc9b9-dbdd-4d42-aca7-e4a47157839c" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_339a81cd-7ca4-47d4-ad9c-5137008799f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_WesternAllianceBankTermLoanAndSecurityAgreementMember_9896a957-8859-4dd6-8611-3c2e51719b90" xlink:href="tdup-20210930.xsd#tdup_WesternAllianceBankTermLoanAndSecurityAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_339a81cd-7ca4-47d4-ad9c-5137008799f6" xlink:to="loc_tdup_WesternAllianceBankTermLoanAndSecurityAgreementMember_9896a957-8859-4dd6-8611-3c2e51719b90" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_2fa8ccd7-33cb-4062-8008-f9d11b97e676" 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_db9eb1c0-3ee0-4f45-9efd-a1a54a873268" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_2fa8ccd7-33cb-4062-8008-f9d11b97e676" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_2fa8ccd7-33cb-4062-8008-f9d11b97e676_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_2fa8ccd7-33cb-4062-8008-f9d11b97e676" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_2fa8ccd7-33cb-4062-8008-f9d11b97e676_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_c63c81af-b826-48fa-aef4-a2632c1c02ba" 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_2fa8ccd7-33cb-4062-8008-f9d11b97e676" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_c63c81af-b826-48fa-aef4-a2632c1c02ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SeriesE1ConvertiblePreferredStockMember_c275964a-3946-410c-b4e4-4d653980ac6f" xlink:href="tdup-20210930.xsd#tdup_SeriesE1ConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_c63c81af-b826-48fa-aef4-a2632c1c02ba" xlink:to="loc_tdup_SeriesE1ConvertiblePreferredStockMember_c275964a-3946-410c-b4e4-4d653980ac6f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ClassBCommonStockWarrantMember_58b217b6-e130-443f-990c-516c9a8ffefb" xlink:href="tdup-20210930.xsd#tdup_ClassBCommonStockWarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_c63c81af-b826-48fa-aef4-a2632c1c02ba" xlink:to="loc_tdup_ClassBCommonStockWarrantMember_58b217b6-e130-443f-990c-516c9a8ffefb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_8d05299f-a40b-4872-a813-32bf107b0833" 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_db9eb1c0-3ee0-4f45-9efd-a1a54a873268" xlink:to="loc_us-gaap_VariableRateAxis_8d05299f-a40b-4872-a813-32bf107b0833" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_8d05299f-a40b-4872-a813-32bf107b0833_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_8d05299f-a40b-4872-a813-32bf107b0833" xlink:to="loc_us-gaap_VariableRateDomain_8d05299f-a40b-4872-a813-32bf107b0833_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_416f952d-f92c-41b0-aedb-54aff93b762f" 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_8d05299f-a40b-4872-a813-32bf107b0833" xlink:to="loc_us-gaap_VariableRateDomain_416f952d-f92c-41b0-aedb-54aff93b762f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrimeRateMember_55a10e15-5a31-4586-8ed9-1ad80bbf58fb" 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_416f952d-f92c-41b0-aedb-54aff93b762f" xlink:to="loc_us-gaap_PrimeRateMember_55a10e15-5a31-4586-8ed9-1ad80bbf58fb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_763e83ef-0a44-48a7-9ba6-6d9ff9fc00ad" 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_db9eb1c0-3ee0-4f45-9efd-a1a54a873268" xlink:to="loc_srt_RangeAxis_763e83ef-0a44-48a7-9ba6-6d9ff9fc00ad" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_763e83ef-0a44-48a7-9ba6-6d9ff9fc00ad_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_763e83ef-0a44-48a7-9ba6-6d9ff9fc00ad" xlink:to="loc_srt_RangeMember_763e83ef-0a44-48a7-9ba6-6d9ff9fc00ad_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_5b8903ab-1df5-4647-8593-be2289e43e12" 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_763e83ef-0a44-48a7-9ba6-6d9ff9fc00ad" xlink:to="loc_srt_RangeMember_5b8903ab-1df5-4647-8593-be2289e43e12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_788807b0-529f-4e14-9fcf-91a706cf75a6" 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_5b8903ab-1df5-4647-8593-be2289e43e12" xlink:to="loc_srt_MinimumMember_788807b0-529f-4e14-9fcf-91a706cf75a6" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/LongtermDebtandConvertiblePreferredStockWarrantsScheduleofWarrantsIssuedDetails" xlink:type="simple" xlink:href="tdup-20210930.xsd#LongtermDebtandConvertiblePreferredStockWarrantsScheduleofWarrantsIssuedDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/LongtermDebtandConvertiblePreferredStockWarrantsScheduleofWarrantsIssuedDetails" xlink:type="extended" id="i71d4439252c4409191c328f2de4d6a61_LongtermDebtandConvertiblePreferredStockWarrantsScheduleofWarrantsIssuedDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_982a7d05-616a-4f47-8291-633c7ea4517d" 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_e912f2a2-2ec7-44bc-906e-2aaf0d28b0b1" 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_982a7d05-616a-4f47-8291-633c7ea4517d" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_e912f2a2-2ec7-44bc-906e-2aaf0d28b0b1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_e61a0e2f-6a7c-4c27-93a5-4bee84e961a3" 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_982a7d05-616a-4f47-8291-633c7ea4517d" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_e61a0e2f-6a7c-4c27-93a5-4bee84e961a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_a605d162-ea6a-400e-8717-62491cc4a435" 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_982a7d05-616a-4f47-8291-633c7ea4517d" xlink:to="loc_us-gaap_DebtInstrumentTable_a605d162-ea6a-400e-8717-62491cc4a435" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_3005ac48-7484-4db7-a343-8a4e6d0a1dd0" 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_a605d162-ea6a-400e-8717-62491cc4a435" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_3005ac48-7484-4db7-a343-8a4e6d0a1dd0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_3005ac48-7484-4db7-a343-8a4e6d0a1dd0_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_3005ac48-7484-4db7-a343-8a4e6d0a1dd0" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_3005ac48-7484-4db7-a343-8a4e6d0a1dd0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_07559030-97fa-4eea-9de6-2000b4dc5714" 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_3005ac48-7484-4db7-a343-8a4e6d0a1dd0" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_07559030-97fa-4eea-9de6-2000b4dc5714" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SeriesDConvertiblePreferredStockIssued1222015Member_7061c4c2-59c4-4ee0-adf7-42a65961985e" xlink:href="tdup-20210930.xsd#tdup_SeriesDConvertiblePreferredStockIssued1222015Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_07559030-97fa-4eea-9de6-2000b4dc5714" xlink:to="loc_tdup_SeriesDConvertiblePreferredStockIssued1222015Member_7061c4c2-59c4-4ee0-adf7-42a65961985e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SeriesDConvertiblePreferredStockIssued4202015Member_af10aedd-021e-4f53-9e93-1b00a84309d6" xlink:href="tdup-20210930.xsd#tdup_SeriesDConvertiblePreferredStockIssued4202015Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_07559030-97fa-4eea-9de6-2000b4dc5714" xlink:to="loc_tdup_SeriesDConvertiblePreferredStockIssued4202015Member_af10aedd-021e-4f53-9e93-1b00a84309d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SeriesE1ConvertiblePreferredStockIssued272019Member_8bd5a041-d9ea-467e-8098-9e6c3aec94bf" xlink:href="tdup-20210930.xsd#tdup_SeriesE1ConvertiblePreferredStockIssued272019Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_07559030-97fa-4eea-9de6-2000b4dc5714" xlink:to="loc_tdup_SeriesE1ConvertiblePreferredStockIssued272019Member_8bd5a041-d9ea-467e-8098-9e6c3aec94bf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SeriesFConvertiblePreferredStockIssued5292020Member_11b41616-1dba-4b81-94a0-ad29f9bb5a18" xlink:href="tdup-20210930.xsd#tdup_SeriesFConvertiblePreferredStockIssued5292020Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_07559030-97fa-4eea-9de6-2000b4dc5714" xlink:to="loc_tdup_SeriesFConvertiblePreferredStockIssued5292020Member_11b41616-1dba-4b81-94a0-ad29f9bb5a18" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SeriesE1ConvertiblePreferredStockIssued8142020Member_d5491663-17eb-47c3-aca1-9ce1a3011557" xlink:href="tdup-20210930.xsd#tdup_SeriesE1ConvertiblePreferredStockIssued8142020Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_07559030-97fa-4eea-9de6-2000b4dc5714" xlink:to="loc_tdup_SeriesE1ConvertiblePreferredStockIssued8142020Member_d5491663-17eb-47c3-aca1-9ce1a3011557" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SeriesE1ConvertiblePreferredStockIssued11252020Member_e25ad16b-0299-4280-ae33-05c06f38ed4b" xlink:href="tdup-20210930.xsd#tdup_SeriesE1ConvertiblePreferredStockIssued11252020Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_07559030-97fa-4eea-9de6-2000b4dc5714" xlink:to="loc_tdup_SeriesE1ConvertiblePreferredStockIssued11252020Member_e25ad16b-0299-4280-ae33-05c06f38ed4b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SeriesE1ConvertiblePreferredStockIssued2282021Member_008063e8-9d33-4e4a-b408-62914e082abd" xlink:href="tdup-20210930.xsd#tdup_SeriesE1ConvertiblePreferredStockIssued2282021Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_07559030-97fa-4eea-9de6-2000b4dc5714" xlink:to="loc_tdup_SeriesE1ConvertiblePreferredStockIssued2282021Member_008063e8-9d33-4e4a-b408-62914e082abd" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/CommonStockandCommonStockWarrantsNarrativeDetails" xlink:type="simple" xlink:href="tdup-20210930.xsd#CommonStockandCommonStockWarrantsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/CommonStockandCommonStockWarrantsNarrativeDetails" xlink:type="extended" id="i4b2ea3b9940c4a53b7b241661e0bcb95_CommonStockandCommonStockWarrantsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_e66b4cf5-a337-4939-96a3-a84dba002a75" 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_84ae4e2b-eef2-4f5a-bf61-8f0350439713" xlink:href="tdup-20210930.xsd#tdup_NumberOfVotesForEachShareOfCommonStockHeld"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_e66b4cf5-a337-4939-96a3-a84dba002a75" xlink:to="loc_tdup_NumberOfVotesForEachShareOfCommonStockHeld_84ae4e2b-eef2-4f5a-bf61-8f0350439713" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_StockholdersEquityConversionOfStockRatio_8ba7e5de-5313-4032-a8bf-8e081b70b5b9" xlink:href="tdup-20210930.xsd#tdup_StockholdersEquityConversionOfStockRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_e66b4cf5-a337-4939-96a3-a84dba002a75" xlink:to="loc_tdup_StockholdersEquityConversionOfStockRatio_8ba7e5de-5313-4032-a8bf-8e081b70b5b9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_e3c26694-9047-4246-ad8c-daa13ff4e578" 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_e66b4cf5-a337-4939-96a3-a84dba002a75" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_e3c26694-9047-4246-ad8c-daa13ff4e578" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_3d1b0002-d195-4354-a9d4-5251ff201bba" 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_e66b4cf5-a337-4939-96a3-a84dba002a75" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_3d1b0002-d195-4354-a9d4-5251ff201bba" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_719a4bff-32a9-470b-8da4-1342f89fe485" 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_e66b4cf5-a337-4939-96a3-a84dba002a75" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_719a4bff-32a9-470b-8da4-1342f89fe485" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_833dc0b1-d0f1-4117-ab38-c2a9e3c78f69" 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_e66b4cf5-a337-4939-96a3-a84dba002a75" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_833dc0b1-d0f1-4117-ab38-c2a9e3c78f69" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_eb722191-c83d-406c-8699-d0973ee768f2" 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_e66b4cf5-a337-4939-96a3-a84dba002a75" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_eb722191-c83d-406c-8699-d0973ee768f2" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfStockIssuanceCosts_5703e0d9-c615-4530-9d11-fa57a8f04b34" 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_e66b4cf5-a337-4939-96a3-a84dba002a75" xlink:to="loc_us-gaap_PaymentsOfStockIssuanceCosts_5703e0d9-c615-4530-9d11-fa57a8f04b34" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_StockIssuedDuringPeriodSharesWarrantsIssued_6922e19b-5a5c-4e44-933c-506e3a377deb" xlink:href="tdup-20210930.xsd#tdup_StockIssuedDuringPeriodSharesWarrantsIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_e66b4cf5-a337-4939-96a3-a84dba002a75" xlink:to="loc_tdup_StockIssuedDuringPeriodSharesWarrantsIssued_6922e19b-5a5c-4e44-933c-506e3a377deb" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_28a9c4fb-3295-4074-902e-1ee8ed16f80b" 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_e66b4cf5-a337-4939-96a3-a84dba002a75" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_28a9c4fb-3295-4074-902e-1ee8ed16f80b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_6c325155-ec17-4202-a981-785ac7487582" 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_28a9c4fb-3295-4074-902e-1ee8ed16f80b" xlink:to="loc_us-gaap_StatementClassOfStockAxis_6c325155-ec17-4202-a981-785ac7487582" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_6c325155-ec17-4202-a981-785ac7487582_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_6c325155-ec17-4202-a981-785ac7487582" xlink:to="loc_us-gaap_ClassOfStockDomain_6c325155-ec17-4202-a981-785ac7487582_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_13d9a5d5-8deb-48b4-9364-5b8cf1c5eaba" 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_6c325155-ec17-4202-a981-785ac7487582" xlink:to="loc_us-gaap_ClassOfStockDomain_13d9a5d5-8deb-48b4-9364-5b8cf1c5eaba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_9db6b132-9265-4b9a-bf30-b68dd3e02d31" 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_13d9a5d5-8deb-48b4-9364-5b8cf1c5eaba" xlink:to="loc_us-gaap_CommonClassAMember_9db6b132-9265-4b9a-bf30-b68dd3e02d31" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_af30d643-adfa-4d24-8dea-6c10fcde3d3f" 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_13d9a5d5-8deb-48b4-9364-5b8cf1c5eaba" xlink:to="loc_us-gaap_CommonClassBMember_af30d643-adfa-4d24-8dea-6c10fcde3d3f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_beaa9e84-2bf2-4fe9-976b-ff313b9cd5eb" 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_28a9c4fb-3295-4074-902e-1ee8ed16f80b" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_beaa9e84-2bf2-4fe9-976b-ff313b9cd5eb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_beaa9e84-2bf2-4fe9-976b-ff313b9cd5eb_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_beaa9e84-2bf2-4fe9-976b-ff313b9cd5eb" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_beaa9e84-2bf2-4fe9-976b-ff313b9cd5eb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_7bd3b0d0-5454-4a1a-adb2-7313fcca9f4d" 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_beaa9e84-2bf2-4fe9-976b-ff313b9cd5eb" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_7bd3b0d0-5454-4a1a-adb2-7313fcca9f4d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ClassBCommonStockWarrantMember_aa0e8ecc-37c3-4458-aee6-d7f5551c049c" xlink:href="tdup-20210930.xsd#tdup_ClassBCommonStockWarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_7bd3b0d0-5454-4a1a-adb2-7313fcca9f4d" xlink:to="loc_tdup_ClassBCommonStockWarrantMember_aa0e8ecc-37c3-4458-aee6-d7f5551c049c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_22170aae-c979-4e3c-ab3c-fa2989b0a022" 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_28a9c4fb-3295-4074-902e-1ee8ed16f80b" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_22170aae-c979-4e3c-ab3c-fa2989b0a022" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_22170aae-c979-4e3c-ab3c-fa2989b0a022_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_22170aae-c979-4e3c-ab3c-fa2989b0a022" xlink:to="loc_us-gaap_EquityComponentDomain_22170aae-c979-4e3c-ab3c-fa2989b0a022_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_6c84f165-3180-42ae-ba14-ec545afc5200" 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_22170aae-c979-4e3c-ab3c-fa2989b0a022" xlink:to="loc_us-gaap_EquityComponentDomain_6c84f165-3180-42ae-ba14-ec545afc5200" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_c5d49aef-5c2d-46ad-9ea1-04b1f5c6dab3" 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_6c84f165-3180-42ae-ba14-ec545afc5200" xlink:to="loc_us-gaap_CommonStockMember_c5d49aef-5c2d-46ad-9ea1-04b1f5c6dab3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_9a46933f-bcc2-4e8c-a520-24881a22f7b4" 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_28a9c4fb-3295-4074-902e-1ee8ed16f80b" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_9a46933f-bcc2-4e8c-a520-24881a22f7b4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_9a46933f-bcc2-4e8c-a520-24881a22f7b4_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_9a46933f-bcc2-4e8c-a520-24881a22f7b4" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_9a46933f-bcc2-4e8c-a520-24881a22f7b4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_053cc7f6-7836-4f03-b331-4b7f745fdf62" 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_9a46933f-bcc2-4e8c-a520-24881a22f7b4" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_053cc7f6-7836-4f03-b331-4b7f745fdf62" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_FollowOnPublicOfferingMember_021e3dfb-536b-4b05-a6e5-aebd0eaca54f" xlink:href="tdup-20210930.xsd#tdup_FollowOnPublicOfferingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_053cc7f6-7836-4f03-b331-4b7f745fdf62" xlink:to="loc_tdup_FollowOnPublicOfferingMember_021e3dfb-536b-4b05-a6e5-aebd0eaca54f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OverAllotmentOptionMember_00137348-8212-4b1f-9f0d-0f3515bd10fc" 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_053cc7f6-7836-4f03-b331-4b7f745fdf62" xlink:to="loc_us-gaap_OverAllotmentOptionMember_00137348-8212-4b1f-9f0d-0f3515bd10fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_b638ae70-ea20-4566-a4a6-60d1bf7d924a" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_28a9c4fb-3295-4074-902e-1ee8ed16f80b" xlink:to="loc_dei_LegalEntityAxis_b638ae70-ea20-4566-a4a6-60d1bf7d924a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_b638ae70-ea20-4566-a4a6-60d1bf7d924a_default" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_b638ae70-ea20-4566-a4a6-60d1bf7d924a" xlink:to="loc_dei_EntityDomain_b638ae70-ea20-4566-a4a6-60d1bf7d924a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_b733dd4f-b644-44c0-8a05-71b43a26c5a2" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_b638ae70-ea20-4566-a4a6-60d1bf7d924a" xlink:to="loc_dei_EntityDomain_b733dd4f-b644-44c0-8a05-71b43a26c5a2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SellingStockholdersMember_4851bf3d-5e63-4fe0-aa0c-5af766d8acf8" xlink:href="tdup-20210930.xsd#tdup_SellingStockholdersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_b733dd4f-b644-44c0-8a05-71b43a26c5a2" xlink:to="loc_tdup_SellingStockholdersMember_4851bf3d-5e63-4fe0-aa0c-5af766d8acf8" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/CommonStockandCommonStockWarrantsScheduleofCommonStockIssuedandOutstandingDetails" xlink:type="simple" xlink:href="tdup-20210930.xsd#CommonStockandCommonStockWarrantsScheduleofCommonStockIssuedandOutstandingDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/CommonStockandCommonStockWarrantsScheduleofCommonStockIssuedandOutstandingDetails" xlink:type="extended" id="ice8f4c8054c0483f8f3c6a0134ba28ca_CommonStockandCommonStockWarrantsScheduleofCommonStockIssuedandOutstandingDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_7ac363f6-8b57-4607-af73-eb3fc59309ac" 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_f4eb731d-e1b5-4baa-881a-4eeddcacb05e" 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_7ac363f6-8b57-4607-af73-eb3fc59309ac" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_f4eb731d-e1b5-4baa-881a-4eeddcacb05e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_79e5e90c-2d9c-4f1a-946d-8e2972de6dae" 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_7ac363f6-8b57-4607-af73-eb3fc59309ac" xlink:to="loc_us-gaap_CommonStockSharesIssued_79e5e90c-2d9c-4f1a-946d-8e2972de6dae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_7dc1042a-26be-4d2c-9487-c64334e0d80d" 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_7ac363f6-8b57-4607-af73-eb3fc59309ac" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_7dc1042a-26be-4d2c-9487-c64334e0d80d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_9bd71fb8-360b-4490-8927-aea16cb8163c" 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_7ac363f6-8b57-4607-af73-eb3fc59309ac" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_9bd71fb8-360b-4490-8927-aea16cb8163c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_1df915a3-8966-4000-81f6-50a9586b0429" 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_9bd71fb8-360b-4490-8927-aea16cb8163c" xlink:to="loc_us-gaap_StatementClassOfStockAxis_1df915a3-8966-4000-81f6-50a9586b0429" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_1df915a3-8966-4000-81f6-50a9586b0429_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_1df915a3-8966-4000-81f6-50a9586b0429" xlink:to="loc_us-gaap_ClassOfStockDomain_1df915a3-8966-4000-81f6-50a9586b0429_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_caabbbab-9427-415a-82aa-ff16a3b3b3fe" 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_1df915a3-8966-4000-81f6-50a9586b0429" xlink:to="loc_us-gaap_ClassOfStockDomain_caabbbab-9427-415a-82aa-ff16a3b3b3fe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_8affe39e-8d9d-45b0-aa1c-6b8bbab202c3" 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_caabbbab-9427-415a-82aa-ff16a3b3b3fe" xlink:to="loc_us-gaap_CommonClassAMember_8affe39e-8d9d-45b0-aa1c-6b8bbab202c3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_a503e4b4-b942-4f32-b25a-630dd6ad30bd" 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_caabbbab-9427-415a-82aa-ff16a3b3b3fe" xlink:to="loc_us-gaap_CommonClassBMember_a503e4b4-b942-4f32-b25a-630dd6ad30bd" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/StockBasedCompensationPlansNarrativeDetails" xlink:type="simple" xlink:href="tdup-20210930.xsd#StockBasedCompensationPlansNarrativeDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/StockBasedCompensationPlansNarrativeDetails" xlink:type="extended" id="i13de50e73277427c9355b53637541d18_StockBasedCompensationPlansNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_f469209e-c5a9-4716-aa11-a40e64be64bf" 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_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_93100887-3c77-468a-95dd-06ca29f09032" 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_f469209e-c5a9-4716-aa11-a40e64be64bf" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_93100887-3c77-468a-95dd-06ca29f09032" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_d155de31-a611-4446-89c0-23ac2433e7ec" 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_f469209e-c5a9-4716-aa11-a40e64be64bf" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_d155de31-a611-4446-89c0-23ac2433e7ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_f160acf6-ff79-4e66-927f-bd11fedb554a" 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_f469209e-c5a9-4716-aa11-a40e64be64bf" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_f160acf6-ff79-4e66-927f-bd11fedb554a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_e4fbc841-3306-4627-bd7b-ba79e8ed130b" 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_f469209e-c5a9-4716-aa11-a40e64be64bf" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_e4fbc841-3306-4627-bd7b-ba79e8ed130b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfTotalAwards_29c25da3-6937-4ff8-99a7-c70e552c9300" xlink:href="tdup-20210930.xsd#tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfTotalAwards"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_f469209e-c5a9-4716-aa11-a40e64be64bf" xlink:to="loc_tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfTotalAwards_29c25da3-6937-4ff8-99a7-c70e552c9300" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_51296771-6b8a-4c3a-b50f-8a86eb16f1e0" 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_f469209e-c5a9-4716-aa11-a40e64be64bf" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_51296771-6b8a-4c3a-b50f-8a86eb16f1e0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardPeriodAfterInitialPublicOfferingToCommenceVesting_97e69f05-b886-4a16-9368-3d1b539368bd" xlink:href="tdup-20210930.xsd#tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardPeriodAfterInitialPublicOfferingToCommenceVesting"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_f469209e-c5a9-4716-aa11-a40e64be64bf" xlink:to="loc_tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardPeriodAfterInitialPublicOfferingToCommenceVesting_97e69f05-b886-4a16-9368-3d1b539368bd" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_b52aa22e-f2f1-425d-ba38-031f96d83c13" 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_f469209e-c5a9-4716-aa11-a40e64be64bf" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_b52aa22e-f2f1-425d-ba38-031f96d83c13" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_cfd629a7-8bf0-484d-8f63-7aa5988ea679" 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_f469209e-c5a9-4716-aa11-a40e64be64bf" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_cfd629a7-8bf0-484d-8f63-7aa5988ea679" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_62e0ae55-c9cc-4573-a26b-cfae8b7b66ce" 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_f469209e-c5a9-4716-aa11-a40e64be64bf" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_62e0ae55-c9cc-4573-a26b-cfae8b7b66ce" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_796530cd-b408-4670-b0ec-bc820ee662d3" 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_f469209e-c5a9-4716-aa11-a40e64be64bf" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_796530cd-b408-4670-b0ec-bc820ee662d3" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_002cfebe-d737-422a-bee9-ec6cd3c6696d" 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_f469209e-c5a9-4716-aa11-a40e64be64bf" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_002cfebe-d737-422a-bee9-ec6cd3c6696d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_125238c6-93a9-414b-83e6-4c1204fcdc2d" 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_002cfebe-d737-422a-bee9-ec6cd3c6696d" xlink:to="loc_us-gaap_AwardTypeAxis_125238c6-93a9-414b-83e6-4c1204fcdc2d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_125238c6-93a9-414b-83e6-4c1204fcdc2d_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_125238c6-93a9-414b-83e6-4c1204fcdc2d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_125238c6-93a9-414b-83e6-4c1204fcdc2d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4e4d0b3b-1366-4ad7-9425-f550b9dda89d" 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_125238c6-93a9-414b-83e6-4c1204fcdc2d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4e4d0b3b-1366-4ad7-9425-f550b9dda89d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_5eae8148-21a6-4b4d-bafc-3ff07605c67f" 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_4e4d0b3b-1366-4ad7-9425-f550b9dda89d" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_5eae8148-21a6-4b4d-bafc-3ff07605c67f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_5c495b92-12e3-4b56-9d33-2c313eb52437" 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_4e4d0b3b-1366-4ad7-9425-f550b9dda89d" xlink:to="loc_us-gaap_EmployeeStockMember_5c495b92-12e3-4b56-9d33-2c313eb52437" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_02f1be2c-68f8-4c7f-b753-e07b5d54288b" 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_4e4d0b3b-1366-4ad7-9425-f550b9dda89d" xlink:to="loc_us-gaap_EmployeeStockOptionMember_02f1be2c-68f8-4c7f-b753-e07b5d54288b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ShareBasedPaymentArrangementOptionVestingFourYearsFromCommencementMember_b39bb68e-1bd0-4cb8-b9f4-892755d8ffa6" xlink:href="tdup-20210930.xsd#tdup_ShareBasedPaymentArrangementOptionVestingFourYearsFromCommencementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeStockOptionMember_02f1be2c-68f8-4c7f-b753-e07b5d54288b" xlink:to="loc_tdup_ShareBasedPaymentArrangementOptionVestingFourYearsFromCommencementMember_b39bb68e-1bd0-4cb8-b9f4-892755d8ffa6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ShareBasedPaymentArrangementOptionFourYearVestingFromIPOMember_03f8fcaa-ab06-4b10-8201-840344109f5f" xlink:href="tdup-20210930.xsd#tdup_ShareBasedPaymentArrangementOptionFourYearVestingFromIPOMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeStockOptionMember_02f1be2c-68f8-4c7f-b753-e07b5d54288b" xlink:to="loc_tdup_ShareBasedPaymentArrangementOptionFourYearVestingFromIPOMember_03f8fcaa-ab06-4b10-8201-840344109f5f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_bee89840-2a88-4716-a036-58f7c68207b0" 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_002cfebe-d737-422a-bee9-ec6cd3c6696d" xlink:to="loc_us-gaap_PlanNameAxis_bee89840-2a88-4716-a036-58f7c68207b0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_bee89840-2a88-4716-a036-58f7c68207b0_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_bee89840-2a88-4716-a036-58f7c68207b0" xlink:to="loc_us-gaap_PlanNameDomain_bee89840-2a88-4716-a036-58f7c68207b0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_89abe708-97de-46b8-b864-8b8b608f7021" 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_bee89840-2a88-4716-a036-58f7c68207b0" xlink:to="loc_us-gaap_PlanNameDomain_89abe708-97de-46b8-b864-8b8b608f7021" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_StockIncentivePlan2021Member_a70de757-c324-47f2-9bc5-2bea467a1576" xlink:href="tdup-20210930.xsd#tdup_StockIncentivePlan2021Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_89abe708-97de-46b8-b864-8b8b608f7021" xlink:to="loc_tdup_StockIncentivePlan2021Member_a70de757-c324-47f2-9bc5-2bea467a1576" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_StockIncentivePlan2010Member_30b58000-ec38-4308-a9db-b1894d8f23ce" xlink:href="tdup-20210930.xsd#tdup_StockIncentivePlan2010Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_89abe708-97de-46b8-b864-8b8b608f7021" xlink:to="loc_tdup_StockIncentivePlan2010Member_30b58000-ec38-4308-a9db-b1894d8f23ce" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/StockBasedCompensationPlansScheduleofStockBasedCompensationExpenseDetails" xlink:type="simple" xlink:href="tdup-20210930.xsd#StockBasedCompensationPlansScheduleofStockBasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/StockBasedCompensationPlansScheduleofStockBasedCompensationExpenseDetails" xlink:type="extended" id="id85a4f3259d3433fa6c438702fb1dd8d_StockBasedCompensationPlansScheduleofStockBasedCompensationExpenseDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7703aac7-59dc-47e2-89d4-40b568d56f8c" 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_cd3b45b9-2be5-432f-a6fb-3a2018e4361b" 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_7703aac7-59dc-47e2-89d4-40b568d56f8c" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_cd3b45b9-2be5-432f-a6fb-3a2018e4361b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_2fdd4893-4872-45f0-b35c-3e7ab16dcff3" 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_7703aac7-59dc-47e2-89d4-40b568d56f8c" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_2fdd4893-4872-45f0-b35c-3e7ab16dcff3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_f38d2b02-979e-429d-9cb3-671dc16237fb" 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_2fdd4893-4872-45f0-b35c-3e7ab16dcff3" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_f38d2b02-979e-429d-9cb3-671dc16237fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_f38d2b02-979e-429d-9cb3-671dc16237fb_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_f38d2b02-979e-429d-9cb3-671dc16237fb" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_f38d2b02-979e-429d-9cb3-671dc16237fb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_5ecea126-7964-46d0-8086-443484a0ea18" 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_f38d2b02-979e-429d-9cb3-671dc16237fb" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_5ecea126-7964-46d0-8086-443484a0ea18" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_OperationsProductAndTechnologyMember_2abc7125-9121-4948-97dd-be360d636b14" xlink:href="tdup-20210930.xsd#tdup_OperationsProductAndTechnologyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_5ecea126-7964-46d0-8086-443484a0ea18" xlink:to="loc_tdup_OperationsProductAndTechnologyMember_2abc7125-9121-4948-97dd-be360d636b14" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_MarketingExpenseMember_a1aca11d-0ddb-4eab-a4ed-5c635b3fd039" xlink:href="tdup-20210930.xsd#tdup_MarketingExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_5ecea126-7964-46d0-8086-443484a0ea18" xlink:to="loc_tdup_MarketingExpenseMember_a1aca11d-0ddb-4eab-a4ed-5c635b3fd039" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_9f858a11-0f8e-41ac-950e-fcc809a18312" 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_5ecea126-7964-46d0-8086-443484a0ea18" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_9f858a11-0f8e-41ac-950e-fcc809a18312" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/NetLossPerShareAttributabletoCommonStockholdersDetails" xlink:type="simple" xlink:href="tdup-20210930.xsd#NetLossPerShareAttributabletoCommonStockholdersDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/NetLossPerShareAttributabletoCommonStockholdersDetails" xlink:type="extended" id="i121915a376b2412eb6ff090390b6b0bd_NetLossPerShareAttributabletoCommonStockholdersDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_8a8cc4fa-45dc-42b4-9b1f-d6350a26b197" 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_14beafa6-3c01-4011-90af-bb7f5b10fd31" 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_8a8cc4fa-45dc-42b4-9b1f-d6350a26b197" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_14beafa6-3c01-4011-90af-bb7f5b10fd31" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_2657e3f4-5939-457f-a506-87de52095e41" 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_8a8cc4fa-45dc-42b4-9b1f-d6350a26b197" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_2657e3f4-5939-457f-a506-87de52095e41" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_afa9a88a-eb95-4ef1-a710-d1e0dcca50e4" 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_2657e3f4-5939-457f-a506-87de52095e41" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_afa9a88a-eb95-4ef1-a710-d1e0dcca50e4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_afa9a88a-eb95-4ef1-a710-d1e0dcca50e4_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_afa9a88a-eb95-4ef1-a710-d1e0dcca50e4" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_afa9a88a-eb95-4ef1-a710-d1e0dcca50e4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_3a1d227c-8722-4347-a45c-b43c10d5d3eb" 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_afa9a88a-eb95-4ef1-a710-d1e0dcca50e4" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_3a1d227c-8722-4347-a45c-b43c10d5d3eb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_4252732d-64b6-43a9-ba2e-a46ccf3c5c10" 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_3a1d227c-8722-4347-a45c-b43c10d5d3eb" xlink:to="loc_us-gaap_EmployeeStockOptionMember_4252732d-64b6-43a9-ba2e-a46ccf3c5c10" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_72e1d7a2-c1de-4682-a04a-509786a0c70a" 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_3a1d227c-8722-4347-a45c-b43c10d5d3eb" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_72e1d7a2-c1de-4682-a04a-509786a0c70a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_6aa814ed-c27e-4960-b946-623fbc78d84e" 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_3a1d227c-8722-4347-a45c-b43c10d5d3eb" xlink:to="loc_us-gaap_EmployeeStockMember_6aa814ed-c27e-4960-b946-623fbc78d84e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertiblePreferredStockMember_566c91f2-9c82-42a5-884e-3463c46ff3ce" 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_3a1d227c-8722-4347-a45c-b43c10d5d3eb" xlink:to="loc_us-gaap_ConvertiblePreferredStockMember_566c91f2-9c82-42a5-884e-3463c46ff3ce" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_49987855-1c54-469b-b227-f656c9a8eef1" 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_3a1d227c-8722-4347-a45c-b43c10d5d3eb" xlink:to="loc_us-gaap_WarrantMember_49987855-1c54-469b-b227-f656c9a8eef1" xlink:type="arc" order="4"/>
  </link:definitionLink>
</link:linkbase>
