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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019e4136-51c6-785b-a734-0ab35dcf4605,g:ee3fbd01-2d10-4082-a7ac-534c74df6f6b-->
<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/Cover" xlink:type="simple" xlink:href="tdup-20260630.xsd#Cover"/>
  <link:definitionLink xlink:role="http://thredup.com/role/Cover" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_019e4136-528c-7a3b-a1fe-f5cffbebe396" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_DocumentInformationTable_019e4136-528c-7a3b-a1fe-f5cffbebe396" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019e4136-528c-7e22-aaca-0d495456b7f2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_019e4136-528c-7a3b-a1fe-f5cffbebe396" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019e4136-528c-7e22-aaca-0d495456b7f2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e4136-528c-7e22-aaca-0d495456b7f2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e4136-528c-7e22-aaca-0d495456b7f2" xlink:to="loc_us-gaap_ClassOfStockDomain_019e4136-528c-7e22-aaca-0d495456b7f2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e4136-528c-7ba3-81e4-d8c6dde8aeed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e4136-528c-7e22-aaca-0d495456b7f2" xlink:to="loc_us-gaap_ClassOfStockDomain_019e4136-528c-7ba3-81e4-d8c6dde8aeed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019e4136-528c-746b-af23-819a03b3b7de" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019e4136-528c-7ba3-81e4-d8c6dde8aeed" xlink:to="loc_us-gaap_CommonClassAMember_019e4136-528c-746b-af23-819a03b3b7de" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_019e4136-528c-7767-b98d-c160f9d8e51f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019e4136-528c-7ba3-81e4-d8c6dde8aeed" xlink:to="loc_us-gaap_CommonClassBMember_019e4136-528c-7767-b98d-c160f9d8e51f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_019e4136-528c-7357-9467-c25774b66fc2" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_DocumentType_019e4136-528c-7357-9467-c25774b66fc2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentQuarterlyReport_019e4136-528c-728d-97dc-86081fb6edc1" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentQuarterlyReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_DocumentQuarterlyReport_019e4136-528c-728d-97dc-86081fb6edc1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_019e4136-528c-76e8-80ad-912c486acef9" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_DocumentPeriodEndDate_019e4136-528c-76e8-80ad-912c486acef9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_019e4136-528c-7494-b6b7-cf043df7b56d" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentTransitionReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_DocumentTransitionReport_019e4136-528c-7494-b6b7-cf043df7b56d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_019e4136-528c-7bae-a1b2-98f49bc686ca" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_EntityFileNumber_019e4136-528c-7bae-a1b2-98f49bc686ca" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_019e4136-528c-7dea-a6a4-179fd3ba45f2" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_EntityRegistrantName_019e4136-528c-7dea-a6a4-179fd3ba45f2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_019e4136-528c-7ec7-9460-eabec6ea6283" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_EntityIncorporationStateCountryCode_019e4136-528c-7ec7-9460-eabec6ea6283" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_019e4136-528c-799f-bbc6-0c21e3413433" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_EntityTaxIdentificationNumber_019e4136-528c-799f-bbc6-0c21e3413433" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_019e4136-528c-7ca3-a06e-77d8974d8cc9" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_EntityAddressAddressLine1_019e4136-528c-7ca3-a06e-77d8974d8cc9" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine2_019e4136-528c-7b6d-b5c1-ced531d03696" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressAddressLine2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_EntityAddressAddressLine2_019e4136-528c-7b6d-b5c1-ced531d03696" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_019e4136-528c-717d-9e44-92934e48e488" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_EntityAddressCityOrTown_019e4136-528c-717d-9e44-92934e48e488" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_019e4136-528c-7f2f-be39-26eff9ee22f9" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_EntityAddressStateOrProvince_019e4136-528c-7f2f-be39-26eff9ee22f9" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_019e4136-528c-7b21-b1b4-f64a49d85918" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_EntityAddressPostalZipCode_019e4136-528c-7b21-b1b4-f64a49d85918" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_019e4136-528c-7935-8c11-ab6da50d1ac1" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_CityAreaCode_019e4136-528c-7935-8c11-ab6da50d1ac1" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_019e4136-528c-72b8-8224-54f78b409b69" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_LocalPhoneNumber_019e4136-528c-72b8-8224-54f78b409b69" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_019e4136-528c-75ce-89c4-8a43883c8539" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_Security12bTitle_019e4136-528c-75ce-89c4-8a43883c8539" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_019e4136-528c-7471-b011-a58cbebdc97f" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_TradingSymbol_019e4136-528c-7471-b011-a58cbebdc97f" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_019e4136-528c-7f1c-aa34-8c1ae5223b39" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_SecurityExchangeName_019e4136-528c-7f1c-aa34-8c1ae5223b39" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_019e4136-528c-7e6c-be6e-ddf4f4160d1a" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityCurrentReportingStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_EntityCurrentReportingStatus_019e4136-528c-7e6c-be6e-ddf4f4160d1a" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_019e4136-528c-7465-8440-0f730716208a" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityInteractiveDataCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_EntityInteractiveDataCurrent_019e4136-528c-7465-8440-0f730716208a" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_019e4136-528c-7aac-87f2-265044f0dc94" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityFilerCategory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_EntityFilerCategory_019e4136-528c-7aac-87f2-265044f0dc94" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_019e4136-528c-7e60-b482-c3058967f321" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntitySmallBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_EntitySmallBusiness_019e4136-528c-7e60-b482-c3058967f321" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_019e4136-528c-70b4-9d42-cd041bd87a27" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_EntityEmergingGrowthCompany_019e4136-528c-70b4-9d42-cd041bd87a27" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityExTransitionPeriod_019e4136-528c-7027-9141-45600b9bcdbd" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityExTransitionPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_EntityExTransitionPeriod_019e4136-528c-7027-9141-45600b9bcdbd" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_019e4136-528c-7c8f-9aa6-4bbf30540f0c" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityShellCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_EntityShellCompany_019e4136-528c-7c8f-9aa6-4bbf30540f0c" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_019e4136-528c-7c9c-8c9c-9133b855afbf" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityCommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_019e4136-528c-7c9c-8c9c-9133b855afbf" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_019e4136-528c-728e-b6b2-a565f1b33c43" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_AmendmentFlag_019e4136-528c-728e-b6b2-a565f1b33c43" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_019e4136-528c-745f-a0d8-60d8a9be27be" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_EntityCentralIndexKey_019e4136-528c-745f-a0d8-60d8a9be27be" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_019e4136-528c-7600-8786-6283e6b84187" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_CurrentFiscalYearEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_CurrentFiscalYearEndDate_019e4136-528c-7600-8786-6283e6b84187" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_019e4136-528c-7099-869a-64801b7016f3" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentFiscalYearFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_DocumentFiscalYearFocus_019e4136-528c-7099-869a-64801b7016f3" xlink:type="arc" order="29"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_019e4136-528c-7896-9869-a78dfacfd526" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentFiscalPeriodFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019e4136-528c-7736-be89-4d9f8af4b644" xlink:to="loc_dei_DocumentFiscalPeriodFocus_019e4136-528c-7896-9869-a78dfacfd526" xlink:type="arc" order="30"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITYUnaudited" xlink:type="simple" xlink:href="tdup-20260630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITYUnaudited"/>
  <link:definitionLink xlink:role="http://thredup.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITYUnaudited" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e4136-528c-7b8f-a8c7-87a614858a81" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e4136-528c-7f36-be6a-df5e23be9de1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e4136-528c-7b8f-a8c7-87a614858a81" xlink:to="loc_us-gaap_StatementTable_019e4136-528c-7f36-be6a-df5e23be9de1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e4136-528c-7cf6-8edd-76612d8d8803" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e4136-528c-7f36-be6a-df5e23be9de1" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e4136-528c-7cf6-8edd-76612d8d8803" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e4136-528c-7cf6-8edd-76612d8d8803_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e4136-528c-7cf6-8edd-76612d8d8803" xlink:to="loc_us-gaap_EquityComponentDomain_019e4136-528c-7cf6-8edd-76612d8d8803_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e4136-528c-74c0-8040-2ae5ad37b9ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e4136-528c-7cf6-8edd-76612d8d8803" xlink:to="loc_us-gaap_EquityComponentDomain_019e4136-528c-74c0-8040-2ae5ad37b9ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019e4136-528c-71d9-892c-1beb0011e2d1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e4136-528c-74c0-8040-2ae5ad37b9ff" xlink:to="loc_us-gaap_CommonStockMember_019e4136-528c-71d9-892c-1beb0011e2d1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019e4136-528c-74f5-83d5-7df1ce0480f5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e4136-528c-74c0-8040-2ae5ad37b9ff" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019e4136-528c-74f5-83d5-7df1ce0480f5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e4136-528c-7499-b0d5-3300c9339130" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e4136-528c-74c0-8040-2ae5ad37b9ff" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e4136-528c-7499-b0d5-3300c9339130" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019e4136-528c-7f26-959c-57b0261375e6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e4136-528c-74c0-8040-2ae5ad37b9ff" xlink:to="loc_us-gaap_RetainedEarningsMember_019e4136-528c-7f26-959c-57b0261375e6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4136-528c-7307-8279-fb07d2b94865" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e4136-528c-7b8f-a8c7-87a614858a81" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4136-528c-7307-8279-fb07d2b94865" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e4136-528c-75c1-9edc-584ffeb92072" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4136-528c-7307-8279-fb07d2b94865" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019e4136-528c-75c1-9edc-584ffeb92072" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e4136-528c-7c4f-b23c-1c4eb679aeff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4136-528c-7307-8279-fb07d2b94865" xlink:to="loc_us-gaap_StockholdersEquity_019e4136-528c-7c4f-b23c-1c4eb679aeff" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019e4136-528c-7f87-b198-8b6230696983" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4136-528c-7307-8279-fb07d2b94865" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019e4136-528c-7f87-b198-8b6230696983" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019e4136-528c-70a1-ae08-63f1d8bf7dd5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4136-528c-7307-8279-fb07d2b94865" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019e4136-528c-70a1-ae08-63f1d8bf7dd5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e4136-528c-7716-9693-ece1dba0875b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4136-528c-7307-8279-fb07d2b94865" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e4136-528c-7716-9693-ece1dba0875b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_019e4136-528c-772c-b079-00c29023f81a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4136-528c-7307-8279-fb07d2b94865" xlink:to="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_019e4136-528c-772c-b079-00c29023f81a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_019e4136-528c-7298-a708-e58c60608d5f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4136-528c-7307-8279-fb07d2b94865" xlink:to="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_019e4136-528c-7298-a708-e58c60608d5f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019e4136-528c-7d72-9cd8-caa3a0c2b52e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4136-528c-7307-8279-fb07d2b94865" xlink:to="loc_us-gaap_NetIncomeLoss_019e4136-528c-7d72-9cd8-caa3a0c2b52e" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019e4136-528c-7de9-9cba-84ef7a80ba56" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e4136-528c-7307-8279-fb07d2b94865" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019e4136-528c-7de9-9cba-84ef7a80ba56" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e4136-528c-7244-82d8-b784f84b6ae7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e4136-528c-78e9-8a1f-ed6fa8718cae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/SignificantAccountingPoliciesNarrativeDetails" xlink:type="simple" xlink:href="tdup-20260630.xsd#SignificantAccountingPoliciesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/SignificantAccountingPoliciesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e4136-528c-75c0-ae5c-21863a8a5599" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019e4136-528c-7e98-959f-aad5acc430a6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e4136-528c-75c0-ae5c-21863a8a5599" xlink:to="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019e4136-528c-7e98-959f-aad5acc430a6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019e4136-528c-7ed4-8b2d-b1ec9058ce3a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019e4136-528c-7e98-959f-aad5acc430a6" xlink:to="loc_srt_ProductOrServiceAxis_019e4136-528c-7ed4-8b2d-b1ec9058ce3a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e4136-528c-7ed4-8b2d-b1ec9058ce3a_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019e4136-528c-7ed4-8b2d-b1ec9058ce3a" xlink:to="loc_srt_ProductsAndServicesDomain_019e4136-528c-7ed4-8b2d-b1ec9058ce3a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019e4136-528c-7306-9b5b-4672aff1793a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019e4136-528c-7ed4-8b2d-b1ec9058ce3a" xlink:to="loc_srt_ProductsAndServicesDomain_019e4136-528c-7306-9b5b-4672aff1793a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_LoyaltyProgramMember_019e4136-528c-7abc-9b1d-c4c9ef8d6ba8" xlink:href="tdup-20260630.xsd#tdup_LoyaltyProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e4136-528c-7306-9b5b-4672aff1793a" xlink:to="loc_tdup_LoyaltyProgramMember_019e4136-528c-7abc-9b1d-c4c9ef8d6ba8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_GiftCardMember_019e4136-528c-70f0-9a0c-8c8d3e84b780" xlink:href="tdup-20260630.xsd#tdup_GiftCardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e4136-528c-7306-9b5b-4672aff1793a" xlink:to="loc_tdup_GiftCardMember_019e4136-528c-70f0-9a0c-8c8d3e84b780" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_GiftCardBreakageMember_019e4136-528c-7979-a4dd-df2f4e180e66" xlink:href="tdup-20260630.xsd#tdup_GiftCardBreakageMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e4136-528c-7306-9b5b-4672aff1793a" xlink:to="loc_tdup_GiftCardBreakageMember_019e4136-528c-7979-a4dd-df2f4e180e66" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SiteCreditMember_019e4136-528c-7969-b51f-3e4a1de4e47a" xlink:href="tdup-20260630.xsd#tdup_SiteCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019e4136-528c-7306-9b5b-4672aff1793a" xlink:to="loc_tdup_SiteCreditMember_019e4136-528c-7969-b51f-3e4a1de4e47a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019e4136-528c-7d0b-a1ae-85abf44da4ec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TimingOfTransferOfGoodOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_019e4136-528c-7e98-959f-aad5acc430a6" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019e4136-528c-7d0b-a1ae-85abf44da4ec" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019e4136-528c-7d0b-a1ae-85abf44da4ec_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019e4136-528c-7d0b-a1ae-85abf44da4ec" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019e4136-528c-7d0b-a1ae-85abf44da4ec_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019e4136-528c-7d4c-b1fd-036be1dc3bce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019e4136-528c-7d0b-a1ae-85abf44da4ec" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019e4136-528c-7d4c-b1fd-036be1dc3bce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredOverTimeMember_019e4136-528c-72fc-9893-799e40d4f3b1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransferredOverTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019e4136-528c-7d4c-b1fd-036be1dc3bce" xlink:to="loc_us-gaap_TransferredOverTimeMember_019e4136-528c-72fc-9893-799e40d4f3b1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfOperatingSegments_019e4136-528c-7668-ac2a-57f0a5fd7e9f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NumberOfOperatingSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e4136-528c-75c0-ae5c-21863a8a5599" xlink:to="loc_us-gaap_NumberOfOperatingSegments_019e4136-528c-7668-ac2a-57f0a5fd7e9f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_019e4136-528c-7e29-af8d-b1b5a674ea1a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NumberOfReportableSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e4136-528c-75c0-ae5c-21863a8a5599" xlink:to="loc_us-gaap_NumberOfReportableSegments_019e4136-528c-7e29-af8d-b1b5a674ea1a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_019e4136-528c-7105-9c83-715de4a70771" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e4136-528c-75c0-ae5c-21863a8a5599" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_019e4136-528c-7105-9c83-715de4a70771" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_019e4136-528c-7eed-9cc4-5898bd56fe3d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiabilityRevenueRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e4136-528c-75c0-ae5c-21863a8a5599" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_019e4136-528c-7eed-9cc4-5898bd56fe3d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ContractWithCustomerLiabilityCurrentConversionPeriod_019e4136-528c-7fc3-8642-3fcf2177370b" xlink:href="tdup-20260630.xsd#tdup_ContractWithCustomerLiabilityCurrentConversionPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e4136-528c-75c0-ae5c-21863a8a5599" xlink:to="loc_tdup_ContractWithCustomerLiabilityCurrentConversionPeriod_019e4136-528c-7fc3-8642-3fcf2177370b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ContractWithCustomerLiabilityRevenueRecognizedIncludingOpeningBalance_019e4136-528c-7f7e-aae1-ab7bbce68fa5" xlink:href="tdup-20260630.xsd#tdup_ContractWithCustomerLiabilityRevenueRecognizedIncludingOpeningBalance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e4136-528c-75c0-ae5c-21863a8a5599" xlink:to="loc_tdup_ContractWithCustomerLiabilityRevenueRecognizedIncludingOpeningBalance_019e4136-528c-7f7e-aae1-ab7bbce68fa5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SellerPayableCurrent_019e4136-528c-77da-909a-07112cdd2ac2" xlink:href="tdup-20260630.xsd#tdup_SellerPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e4136-528c-75c0-ae5c-21863a8a5599" xlink:to="loc_tdup_SellerPayableCurrent_019e4136-528c-77da-909a-07112cdd2ac2" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_SellerPayableLiabilityCurrentRevenueRecognized_019e4136-528c-7c24-b8af-a0d63a76d334" xlink:href="tdup-20260630.xsd#tdup_SellerPayableLiabilityCurrentRevenueRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_019e4136-528c-75c0-ae5c-21863a8a5599" xlink:to="loc_tdup_SellerPayableLiabilityCurrentRevenueRecognized_019e4136-528c-7c24-b8af-a0d63a76d334" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofFinancialInstrumentsMeasuredatFairValueDetails" xlink:type="simple" xlink:href="tdup-20260630.xsd#FinancialInstrumentsandFairValueMeasurementsScheduleofFinancialInstrumentsMeasuredatFairValueDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofFinancialInstrumentsMeasuredatFairValueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e4136-528c-7ce7-841b-e9e0b78a5d48" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e4136-528c-7379-a7b0-f6c819d59590" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e4136-528c-7ce7-841b-e9e0b78a5d48" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e4136-528c-7379-a7b0-f6c819d59590" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4136-528c-7f36-b5df-16dccb5d809c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e4136-528c-7379-a7b0-f6c819d59590" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4136-528c-7f36-b5df-16dccb5d809c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4136-528c-7f36-b5df-16dccb5d809c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4136-528c-7f36-b5df-16dccb5d809c" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4136-528c-7f36-b5df-16dccb5d809c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4136-528c-7476-bd97-9c82db534449" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e4136-528c-7f36-b5df-16dccb5d809c" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4136-528c-7476-bd97-9c82db534449" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019e4136-528c-7fe3-ae30-4ff98c526920" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4136-528c-7476-bd97-9c82db534449" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019e4136-528c-7fe3-ae30-4ff98c526920" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019e4136-528c-7ae1-bfbf-50dcd5034627" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e4136-528c-7476-bd97-9c82db534449" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019e4136-528c-7ae1-bfbf-50dcd5034627" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_019e4136-528c-7735-8838-048526a283ae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e4136-528c-7379-a7b0-f6c819d59590" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_019e4136-528c-7735-8838-048526a283ae" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e4136-528c-7735-8838-048526a283ae_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019e4136-528c-7735-8838-048526a283ae" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e4136-528c-7735-8838-048526a283ae_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e4136-528c-742f-9b4e-66d3885d93f2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019e4136-528c-7735-8838-048526a283ae" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e4136-528c-742f-9b4e-66d3885d93f2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_019e4136-528c-7c02-b140-52f0c73ecc88" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e4136-528c-742f-9b4e-66d3885d93f2" xlink:to="loc_us-gaap_MoneyMarketFundsMember_019e4136-528c-7c02-b140-52f0c73ecc88" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_019e4136-528c-7b31-adce-ca57a76d6be5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e4136-528c-742f-9b4e-66d3885d93f2" xlink:to="loc_us-gaap_CommercialPaperMember_019e4136-528c-7b31-adce-ca57a76d6be5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019e4136-528c-7211-a5a7-512845665c5b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e4136-528c-742f-9b4e-66d3885d93f2" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019e4136-528c-7211-a5a7-512845665c5b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019e4136-528c-7608-bf96-1e692da0c311" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e4136-528c-742f-9b4e-66d3885d93f2" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019e4136-528c-7608-bf96-1e692da0c311" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_019e4136-528c-7fd0-9f20-ccc44cb701cc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasurySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e4136-528c-742f-9b4e-66d3885d93f2" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_019e4136-528c-7fd0-9f20-ccc44cb701cc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019e4136-528c-7a9d-a53b-43516ebfa41e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e4136-528c-7379-a7b0-f6c819d59590" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019e4136-528c-7a9d-a53b-43516ebfa41e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_019e4136-528c-7a9d-a53b-43516ebfa41e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019e4136-528c-7a9d-a53b-43516ebfa41e" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_019e4136-528c-7a9d-a53b-43516ebfa41e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_019e4136-528c-79d0-8370-db3ba549fad4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019e4136-528c-7a9d-a53b-43516ebfa41e" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_019e4136-528c-79d0-8370-db3ba549fad4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_019e4136-528c-7506-b5ed-06c16eda256b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_019e4136-528c-79d0-8370-db3ba549fad4" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_019e4136-528c-7506-b5ed-06c16eda256b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019e4136-528c-7664-aaef-49a50778a294" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e4136-528c-7379-a7b0-f6c819d59590" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019e4136-528c-7664-aaef-49a50778a294" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e4136-528c-7664-aaef-49a50778a294_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e4136-528c-7664-aaef-49a50778a294" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e4136-528c-7664-aaef-49a50778a294_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e4136-528c-7e84-980d-845ce0c3907e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e4136-528c-7664-aaef-49a50778a294" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e4136-528c-7e84-980d-845ce0c3907e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_019e4136-528c-7fb2-8c05-f1c59245f9a9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasurySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e4136-528c-7e84-980d-845ce0c3907e" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_019e4136-528c-7fb2-8c05-f1c59245f9a9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019e4136-528c-79a7-b18c-1de5cd91460d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e4136-528c-7e84-980d-845ce0c3907e" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019e4136-528c-79a7-b18c-1de5cd91460d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019e4136-528c-75ff-a0a1-b2e431cd4c56" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e4136-528c-7e84-980d-845ce0c3907e" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019e4136-528c-75ff-a0a1-b2e431cd4c56" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosureAbstract_019e4136-528c-77ec-b11d-6a1db3824c72" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsFairValueDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e4136-528c-7ce7-841b-e9e0b78a5d48" xlink:to="loc_us-gaap_AssetsFairValueDisclosureAbstract_019e4136-528c-77ec-b11d-6a1db3824c72" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAbstract_019e4136-528c-7f1c-8a35-1273422dc0c8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_019e4136-528c-77ec-b11d-6a1db3824c72" xlink:to="loc_us-gaap_CashAndCashEquivalentsAbstract_019e4136-528c-7f1c-8a35-1273422dc0c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019e4136-528c-78c0-9a31-eb18d1564839" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsAbstract_019e4136-528c-7f1c-8a35-1273422dc0c8" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019e4136-528c-78c0-9a31-eb18d1564839" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesAbstract_019e4136-528c-7e67-8b0f-d1af4d27fec3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_019e4136-528c-77ec-b11d-6a1db3824c72" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesAbstract_019e4136-528c-7e67-8b0f-d1af4d27fec3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_019e4136-528c-70f7-a55b-fdd62fd8920d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesAbstract_019e4136-528c-7e67-8b0f-d1af4d27fec3" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_019e4136-528c-70f7-a55b-fdd62fd8920d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_019e4136-528c-73f4-b8e5-68b5a3302357" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_019e4136-528c-77ec-b11d-6a1db3824c72" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_019e4136-528c-73f4-b8e5-68b5a3302357" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofMarketableSecuritiesbyContractualMaturitiesDetails" xlink:type="simple" xlink:href="tdup-20260630.xsd#FinancialInstrumentsandFairValueMeasurementsScheduleofMarketableSecuritiesbyContractualMaturitiesDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofMarketableSecuritiesbyContractualMaturitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e4136-528c-7aac-b149-d09ebad512fe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e4136-528c-7dc3-b88f-c722939fcb0b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e4136-528c-7aac-b149-d09ebad512fe" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e4136-528c-7dc3-b88f-c722939fcb0b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019e4136-528c-70bf-b76f-ff4712d674d8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e4136-528c-7dc3-b88f-c722939fcb0b" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019e4136-528c-70bf-b76f-ff4712d674d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e4136-528c-70bf-b76f-ff4712d674d8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e4136-528c-70bf-b76f-ff4712d674d8" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e4136-528c-70bf-b76f-ff4712d674d8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e4136-528c-7ec7-b95e-f7b683094e7a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e4136-528c-70bf-b76f-ff4712d674d8" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e4136-528c-7ec7-b95e-f7b683094e7a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_019e4136-528c-7c93-a68f-701cd84738d8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasurySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e4136-528c-7ec7-b95e-f7b683094e7a" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_019e4136-528c-7c93-a68f-701cd84738d8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019e4136-528c-7277-90a2-ab808c668822" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e4136-528c-7ec7-b95e-f7b683094e7a" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019e4136-528c-7277-90a2-ab808c668822" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019e4136-528c-793a-9dc2-7924f355f4c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e4136-528c-7ec7-b95e-f7b683094e7a" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019e4136-528c-793a-9dc2-7924f355f4c6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_019e4136-528c-7dce-af55-8ddff1e6289b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e4136-528c-7aac-b149-d09ebad512fe" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_019e4136-528c-7dce-af55-8ddff1e6289b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue_019e4136-528c-782f-bf8c-e42f97315e1f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e4136-528c-7aac-b149-d09ebad512fe" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue_019e4136-528c-782f-bf8c-e42f97315e1f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_019e4136-528c-7dbd-87ff-d4fe9b55d158" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e4136-528c-7aac-b149-d09ebad512fe" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_019e4136-528c-7dbd-87ff-d4fe9b55d158" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/FinancialInstrumentsandFairValueMeasurementsNarrativeDetails" xlink:type="simple" xlink:href="tdup-20260630.xsd#FinancialInstrumentsandFairValueMeasurementsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/FinancialInstrumentsandFairValueMeasurementsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e4136-528c-78d2-b84b-bd042466a3a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e4136-528c-7ecd-aecf-df6f12a7b1aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e4136-528c-78d2-b84b-bd042466a3a4" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e4136-528c-7ecd-aecf-df6f12a7b1aa" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationAxis_019e4136-528c-7155-8b8b-d100e44c7c92" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisposalGroupClassificationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e4136-528c-7ecd-aecf-df6f12a7b1aa" xlink:to="loc_us-gaap_DisposalGroupClassificationAxis_019e4136-528c-7155-8b8b-d100e44c7c92" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_019e4136-528c-7155-8b8b-d100e44c7c92_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_019e4136-528c-7155-8b8b-d100e44c7c92" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_019e4136-528c-7155-8b8b-d100e44c7c92_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_019e4136-528c-797d-aef8-2dcb4a70a14e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_019e4136-528c-7155-8b8b-d100e44c7c92" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_019e4136-528c-797d-aef8-2dcb4a70a14e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationsDisposedOfBySaleMember_019e4136-528c-75f9-a9c2-401ce4363577" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DiscontinuedOperationsDisposedOfBySaleMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupClassificationDomain_019e4136-528c-797d-aef8-2dcb4a70a14e" xlink:to="loc_us-gaap_DiscontinuedOperationsDisposedOfBySaleMember_019e4136-528c-75f9-a9c2-401ce4363577" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019e4136-528c-7e89-8886-e94206ee0298" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e4136-528c-7ecd-aecf-df6f12a7b1aa" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019e4136-528c-7e89-8886-e94206ee0298" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019e4136-528c-7e89-8886-e94206ee0298_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019e4136-528c-7e89-8886-e94206ee0298" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019e4136-528c-7e89-8886-e94206ee0298_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019e4136-528c-7acc-afbd-96639e30d096" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019e4136-528c-7e89-8886-e94206ee0298" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019e4136-528c-7acc-afbd-96639e30d096" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_RemixMember_019e4136-528c-7761-8d61-cb94f60d7faf" xlink:href="tdup-20260630.xsd#tdup_RemixMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019e4136-528c-7acc-afbd-96639e30d096" xlink:to="loc_tdup_RemixMember_019e4136-528c-7761-8d61-cb94f60d7faf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_DiscontinuedOperationCostMethodInvestmentRetainedAfterDisposalOwnershipInterestDisposed_019e4136-528c-70ed-9ef1-b4002e034a6f" xlink:href="tdup-20260630.xsd#tdup_DiscontinuedOperationCostMethodInvestmentRetainedAfterDisposalOwnershipInterestDisposed"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e4136-528c-78d2-b84b-bd042466a3a4" xlink:to="loc_tdup_DiscontinuedOperationCostMethodInvestmentRetainedAfterDisposalOwnershipInterestDisposed_019e4136-528c-70ed-9ef1-b4002e034a6f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueUpwardPriceAdjustmentAnnualAmount_019e4136-528c-7fd1-8d38-0c999d896e64" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueUpwardPriceAdjustmentAnnualAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e4136-528c-78d2-b84b-bd042466a3a4" xlink:to="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueUpwardPriceAdjustmentAnnualAmount_019e4136-528c-7fd1-8d38-0c999d896e64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueDownwardPriceAdjustmentAnnualAmount_019e4136-528c-7996-83d6-a917c61dae5c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueDownwardPriceAdjustmentAnnualAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e4136-528c-78d2-b84b-bd042466a3a4" xlink:to="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueDownwardPriceAdjustmentAnnualAmount_019e4136-528c-7996-83d6-a917c61dae5c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLossAnnualAmount_019e4136-528c-79a4-9367-704441234f43" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLossAnnualAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e4136-528c-78d2-b84b-bd042466a3a4" xlink:to="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLossAnnualAmount_019e4136-528c-79a4-9367-704441234f43" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount_019e4136-528c-7764-bfc0-9bf7cd42dc69" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e4136-528c-78d2-b84b-bd042466a3a4" xlink:to="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount_019e4136-528c-7764-bfc0-9bf7cd42dc69" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueUpwardPriceAdjustmentCumulativeAmount_019e4136-528c-7708-8ad0-546d67771bdb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueUpwardPriceAdjustmentCumulativeAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e4136-528c-78d2-b84b-bd042466a3a4" xlink:to="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueUpwardPriceAdjustmentCumulativeAmount_019e4136-528c-7708-8ad0-546d67771bdb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueDownwardPriceAdjustmentCumulativeAmount_019e4136-528c-73a2-8ae1-a10b0e604e57" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueDownwardPriceAdjustmentCumulativeAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e4136-528c-78d2-b84b-bd042466a3a4" xlink:to="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueDownwardPriceAdjustmentCumulativeAmount_019e4136-528c-73a2-8ae1-a10b0e604e57" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLossCumulativeAmount_019e4136-528c-7762-b2b9-99a48de1f6ab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLossCumulativeAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e4136-528c-78d2-b84b-bd042466a3a4" xlink:to="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLossCumulativeAmount_019e4136-528c-7762-b2b9-99a48de1f6ab" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/PropertyandEquipmentNetScheduleofPropertyandEquipmentNetDetails" xlink:type="simple" xlink:href="tdup-20260630.xsd#PropertyandEquipmentNetScheduleofPropertyandEquipmentNetDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/PropertyandEquipmentNetScheduleofPropertyandEquipmentNetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e4136-528d-77f2-95a3-f3646d6ab484" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019e4136-528d-77ba-b21b-c340d75bd62e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e4136-528d-77f2-95a3-f3646d6ab484" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019e4136-528d-77ba-b21b-c340d75bd62e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e4136-528d-7c6d-ad04-a92618361886" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019e4136-528d-77ba-b21b-c340d75bd62e" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e4136-528d-7c6d-ad04-a92618361886" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4136-528d-7c6d-ad04-a92618361886_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e4136-528d-7c6d-ad04-a92618361886" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4136-528d-7c6d-ad04-a92618361886_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4136-528d-7482-bba6-2add541c50c5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e4136-528d-7c6d-ad04-a92618361886" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4136-528d-7482-bba6-2add541c50c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MachineryAndEquipmentMember_019e4136-528d-7025-93a4-327a5c909720" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MachineryAndEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4136-528d-7482-bba6-2add541c50c5" xlink:to="loc_us-gaap_MachineryAndEquipmentMember_019e4136-528d-7025-93a4-327a5c909720" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_019e4136-528d-71de-8e34-7c0218e14171" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4136-528d-7482-bba6-2add541c50c5" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_019e4136-528d-71de-8e34-7c0218e14171" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_019fb3cf-3bbf-78e9-91b6-63e9dcab407d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SoftwareAndSoftwareDevelopmentCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4136-528d-7482-bba6-2add541c50c5" xlink:to="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_019fb3cf-3bbf-78e9-91b6-63e9dcab407d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ComputerEquipmentAndSoftwareMember_019e4136-528d-7673-a58b-5a07e5d306ac" xlink:href="tdup-20260630.xsd#tdup_ComputerEquipmentAndSoftwareMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4136-528d-7482-bba6-2add541c50c5" xlink:to="loc_tdup_ComputerEquipmentAndSoftwareMember_019e4136-528d-7673-a58b-5a07e5d306ac" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConstructionInProgressMember_019e4136-528d-78f7-a50b-00a623faa5d1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConstructionInProgressMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4136-528d-7482-bba6-2add541c50c5" xlink:to="loc_us-gaap_ConstructionInProgressMember_019e4136-528d-78f7-a50b-00a623faa5d1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_019e4136-528d-7301-8b47-a5b71c5099ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4136-528d-7482-bba6-2add541c50c5" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_019e4136-528d-7301-8b47-a5b71c5099ff" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_019e4136-528d-7be3-b085-3012fa176266" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e4136-528d-77f2-95a3-f3646d6ab484" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_019e4136-528d-7be3-b085-3012fa176266" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_019e4136-528d-7a3f-b9fc-e97dd4c654a8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e4136-528d-77f2-95a3-f3646d6ab484" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_019e4136-528d-7a3f-b9fc-e97dd4c654a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_019e4136-528d-7f6f-9541-6e9d83e25d78" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e4136-528d-77f2-95a3-f3646d6ab484" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_019e4136-528d-7f6f-9541-6e9d83e25d78" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/LongTermDebtNarrativeDetails" xlink:type="simple" xlink:href="tdup-20260630.xsd#LongTermDebtNarrativeDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/LongTermDebtNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e4136-528d-7cc0-835d-09410c97ba44" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e4136-528d-7bbf-9abd-bc933afa13c7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e4136-528d-7cc0-835d-09410c97ba44" xlink:to="loc_us-gaap_DebtInstrumentTable_019e4136-528d-7bbf-9abd-bc933afa13c7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e4136-528d-7f68-9d02-6c788e9bda9b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e4136-528d-7bbf-9abd-bc933afa13c7" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e4136-528d-7f68-9d02-6c788e9bda9b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e4136-528d-7f68-9d02-6c788e9bda9b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e4136-528d-7f68-9d02-6c788e9bda9b" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e4136-528d-7f68-9d02-6c788e9bda9b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e4136-528d-74b3-b9d8-4df3ea431aea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e4136-528d-7f68-9d02-6c788e9bda9b" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e4136-528d-74b3-b9d8-4df3ea431aea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_WesternAllianceBankTermLoanAndSecurityAgreementMember_019e4136-528d-7392-81c6-07d978ad65e9" xlink:href="tdup-20260630.xsd#tdup_WesternAllianceBankTermLoanAndSecurityAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e4136-528d-74b3-b9d8-4df3ea431aea" xlink:to="loc_tdup_WesternAllianceBankTermLoanAndSecurityAgreementMember_019e4136-528d-7392-81c6-07d978ad65e9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e4136-528d-7c4b-bbd3-1a2232f3aa4a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e4136-528d-7bbf-9abd-bc933afa13c7" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e4136-528d-7c4b-bbd3-1a2232f3aa4a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e4136-528d-7c4b-bbd3-1a2232f3aa4a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e4136-528d-7c4b-bbd3-1a2232f3aa4a" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e4136-528d-7c4b-bbd3-1a2232f3aa4a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e4136-528d-7a71-a53b-6e779eb80f8d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e4136-528d-7c4b-bbd3-1a2232f3aa4a" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e4136-528d-7a71-a53b-6e779eb80f8d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MediumTermNotesMember_019e4136-528d-71a8-8a78-fbea7f4830dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MediumTermNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e4136-528d-7a71-a53b-6e779eb80f8d" xlink:to="loc_us-gaap_MediumTermNotesMember_019e4136-528d-71a8-8a78-fbea7f4830dd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e4136-528d-7cb1-b7e9-b92283ba6858" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e4136-528d-7bbf-9abd-bc933afa13c7" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e4136-528d-7cb1-b7e9-b92283ba6858" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4136-528d-7cb1-b7e9-b92283ba6858_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e4136-528d-7cb1-b7e9-b92283ba6858" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4136-528d-7cb1-b7e9-b92283ba6858_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4136-528d-7a5e-9865-8eb67f05f4bd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e4136-528d-7cb1-b7e9-b92283ba6858" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4136-528d-7a5e-9865-8eb67f05f4bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquipmentMember_019e4136-528d-7455-a600-ea62ca776a0e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e4136-528d-7a5e-9865-8eb67f05f4bd" xlink:to="loc_us-gaap_EquipmentMember_019e4136-528d-7455-a600-ea62ca776a0e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_DebtInstrumentMaximumBorrowingCapacity_019e4136-528d-7535-9b5a-58a4e9f7bd0c" xlink:href="tdup-20260630.xsd#tdup_DebtInstrumentMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e4136-528d-7cc0-835d-09410c97ba44" xlink:to="loc_tdup_DebtInstrumentMaximumBorrowingCapacity_019e4136-528d-7535-9b5a-58a4e9f7bd0c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_DebtInstrumentFloorPercentage_019e4136-528d-7682-8c1f-7609a96080d4" xlink:href="tdup-20260630.xsd#tdup_DebtInstrumentFloorPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e4136-528d-7cc0-835d-09410c97ba44" xlink:to="loc_tdup_DebtInstrumentFloorPercentage_019e4136-528d-7682-8c1f-7609a96080d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019e4136-528d-7297-86a9-04b6e216ed76" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e4136-528d-7cc0-835d-09410c97ba44" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019e4136-528d-7297-86a9-04b6e216ed76" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019e4136-528d-7319-909b-ba46587ad18d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e4136-528d-7cc0-835d-09410c97ba44" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019e4136-528d-7319-909b-ba46587ad18d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebt_019e4136-528d-7e90-87fb-30a7dd21b600" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e4136-528d-7cc0-835d-09410c97ba44" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebt_019e4136-528d-7e90-87fb-30a7dd21b600" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfDebt_019e4136-528d-7073-a32d-730afd4a84c5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RepaymentsOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e4136-528d-7cc0-835d-09410c97ba44" xlink:to="loc_us-gaap_RepaymentsOfDebt_019e4136-528d-7073-a32d-730afd4a84c5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019e4136-528d-7fd9-b27f-ec0c70dbdd39" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e4136-528d-7cc0-835d-09410c97ba44" xlink:to="loc_us-gaap_LongTermDebt_019e4136-528d-7fd9-b27f-ec0c70dbdd39" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestCostsIncurred_019e4136-528d-7a49-9cb9-4acc92169ade" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestCostsIncurred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e4136-528d-7cc0-835d-09410c97ba44" xlink:to="loc_us-gaap_InterestCostsIncurred_019e4136-528d-7a49-9cb9-4acc92169ade" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/CommonStockandStockholdersEquityNarrativeDetails" xlink:type="simple" xlink:href="tdup-20260630.xsd#CommonStockandStockholdersEquityNarrativeDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/CommonStockandStockholdersEquityNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_019e4136-528d-7a7a-b1eb-43204c030dea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_019e4136-528d-7928-b3e2-e63672223126" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e4136-528d-7a7a-b1eb-43204c030dea" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_019e4136-528d-7928-b3e2-e63672223126" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019e4136-528d-7a50-bd03-900801eb4d37" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019e4136-528d-7928-b3e2-e63672223126" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019e4136-528d-7a50-bd03-900801eb4d37" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e4136-528d-7a50-bd03-900801eb4d37_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e4136-528d-7a50-bd03-900801eb4d37" xlink:to="loc_us-gaap_ClassOfStockDomain_019e4136-528d-7a50-bd03-900801eb4d37_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e4136-528d-7db4-81da-2ae1b8febb26" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e4136-528d-7a50-bd03-900801eb4d37" xlink:to="loc_us-gaap_ClassOfStockDomain_019e4136-528d-7db4-81da-2ae1b8febb26" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019e4136-528d-71a3-9db7-44af06a2a490" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019e4136-528d-7db4-81da-2ae1b8febb26" xlink:to="loc_us-gaap_CommonClassAMember_019e4136-528d-71a3-9db7-44af06a2a490" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_019e4136-528d-72e0-bfc6-c9cfe6bc6b70" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019e4136-528d-7db4-81da-2ae1b8febb26" xlink:to="loc_us-gaap_CommonClassBMember_019e4136-528d-72e0-bfc6-c9cfe6bc6b70" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_NumberOfVotesForEachShareOfCommonStockHeld_019e4136-528d-765c-bdf6-7643bf24acd4" xlink:href="tdup-20260630.xsd#tdup_NumberOfVotesForEachShareOfCommonStockHeld"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e4136-528d-7a7a-b1eb-43204c030dea" xlink:to="loc_tdup_NumberOfVotesForEachShareOfCommonStockHeld_019e4136-528d-765c-bdf6-7643bf24acd4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_StockholdersEquityConversionOfStockRatio_019e4136-528d-7b10-a136-659ace422c5b" xlink:href="tdup-20260630.xsd#tdup_StockholdersEquityConversionOfStockRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e4136-528d-7a7a-b1eb-43204c030dea" xlink:to="loc_tdup_StockholdersEquityConversionOfStockRatio_019e4136-528d-7b10-a136-659ace422c5b" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/CommonStockandStockholdersEquityScheduleofCommonStockIssuedandOutstandingDetails" xlink:type="simple" xlink:href="tdup-20260630.xsd#CommonStockandStockholdersEquityScheduleofCommonStockIssuedandOutstandingDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/CommonStockandStockholdersEquityScheduleofCommonStockIssuedandOutstandingDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_019e4136-528d-7460-83cb-9aae19faefb4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_019e4136-528d-7f6a-939e-40bf79939ceb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e4136-528d-7460-83cb-9aae19faefb4" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_019e4136-528d-7f6a-939e-40bf79939ceb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019e4136-528d-7b4d-ad19-5632be828e96" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019e4136-528d-7f6a-939e-40bf79939ceb" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019e4136-528d-7b4d-ad19-5632be828e96" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e4136-528d-7b4d-ad19-5632be828e96_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e4136-528d-7b4d-ad19-5632be828e96" xlink:to="loc_us-gaap_ClassOfStockDomain_019e4136-528d-7b4d-ad19-5632be828e96_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019e4136-528d-7260-94f6-ff9883b62684" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019e4136-528d-7b4d-ad19-5632be828e96" xlink:to="loc_us-gaap_ClassOfStockDomain_019e4136-528d-7260-94f6-ff9883b62684" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019e4136-528d-72e3-8643-1e8903e7ac16" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019e4136-528d-7260-94f6-ff9883b62684" xlink:to="loc_us-gaap_CommonClassAMember_019e4136-528d-72e3-8643-1e8903e7ac16" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_019e4136-528d-749c-a7b3-01a70a4a6d25" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019e4136-528d-7260-94f6-ff9883b62684" xlink:to="loc_us-gaap_CommonClassBMember_019e4136-528d-749c-a7b3-01a70a4a6d25" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_019e4136-528d-7e7f-94eb-806ac293f45e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e4136-528d-7460-83cb-9aae19faefb4" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_019e4136-528d-7e7f-94eb-806ac293f45e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_019e4136-528d-73c3-b6a2-b6ee78d75fe6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e4136-528d-7460-83cb-9aae19faefb4" xlink:to="loc_us-gaap_CommonStockSharesIssued_019e4136-528d-73c3-b6a2-b6ee78d75fe6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e4136-528d-798a-8288-4fc3e2153e56" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e4136-528d-7460-83cb-9aae19faefb4" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019e4136-528d-798a-8288-4fc3e2153e56" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/StockBasedCompensationScheduleofStockBasedCompensationExpenseDetails" xlink:type="simple" xlink:href="tdup-20260630.xsd#StockBasedCompensationScheduleofStockBasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/StockBasedCompensationScheduleofStockBasedCompensationExpenseDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4136-528d-70d9-938a-9c95c72dd342" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4136-528d-78f9-9477-2a19ab7c12df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4136-528d-70d9-938a-9c95c72dd342" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4136-528d-78f9-9477-2a19ab7c12df" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e4136-528d-7718-8852-a960736b9cd0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfIncomeLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4136-528d-78f9-9477-2a19ab7c12df" xlink:to="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e4136-528d-7718-8852-a960736b9cd0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e4136-528d-7432-8e7c-d136b1232728" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4136-528d-70d9-938a-9c95c72dd342" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e4136-528d-7432-8e7c-d136b1232728" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/StockBasedCompensationScheduleofStockBasedCompensationExpenseDetails_1" xlink:type="simple" xlink:href="tdup-20260630.xsd#StockBasedCompensationScheduleofStockBasedCompensationExpenseDetails_1"/>
  <link:definitionLink xlink:role="http://thredup.com/role/StockBasedCompensationScheduleofStockBasedCompensationExpenseDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4136-528d-70d9-938a-9c95c72dd342" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4136-528d-78f9-9477-2a19ab7c12df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4136-528d-70d9-938a-9c95c72dd342" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4136-528d-78f9-9477-2a19ab7c12df" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e4136-528d-7432-8e7c-d136b1232728" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4136-528d-70d9-938a-9c95c72dd342" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e4136-528d-7432-8e7c-d136b1232728" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/StockBasedCompensationNarrativeDetails" xlink:type="simple" xlink:href="tdup-20260630.xsd#StockBasedCompensationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/StockBasedCompensationNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4136-528d-741c-9b8a-eea2ba016ae7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4136-528d-7868-9585-3f36f2e83997" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4136-528d-741c-9b8a-eea2ba016ae7" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4136-528d-7868-9585-3f36f2e83997" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e4136-528d-763c-bf6d-66eceebe6a1a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4136-528d-7868-9585-3f36f2e83997" xlink:to="loc_us-gaap_AwardTypeAxis_019e4136-528d-763c-bf6d-66eceebe6a1a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4136-528d-763c-bf6d-66eceebe6a1a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e4136-528d-763c-bf6d-66eceebe6a1a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4136-528d-763c-bf6d-66eceebe6a1a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4136-528d-7e30-a6a7-993b3ae0dc4f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e4136-528d-763c-bf6d-66eceebe6a1a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4136-528d-7e30-a6a7-993b3ae0dc4f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e4136-528d-7cbd-8927-79889d83e862" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4136-528d-7e30-a6a7-993b3ae0dc4f" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e4136-528d-7cbd-8927-79889d83e862" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_RestrictedStockUnitsRSUsAndPerformanceStockUnitsPSUsMember_019f607e-ce46-7ebd-9c56-41f524fad499" xlink:href="tdup-20260630.xsd#tdup_RestrictedStockUnitsRSUsAndPerformanceStockUnitsPSUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4136-528d-7e30-a6a7-993b3ae0dc4f" xlink:to="loc_tdup_RestrictedStockUnitsRSUsAndPerformanceStockUnitsPSUsMember_019f607e-ce46-7ebd-9c56-41f524fad499" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019f6066-cd1a-7c12-a132-4679167fe16f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4136-528d-7e30-a6a7-993b3ae0dc4f" xlink:to="loc_us-gaap_PerformanceSharesMember_019f6066-cd1a-7c12-a132-4679167fe16f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount_019e4136-528d-7c04-9513-49ead2a86cc3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4136-528d-741c-9b8a-eea2ba016ae7" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount_019e4136-528d-7c04-9513-49ead2a86cc3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_019e4136-528d-7631-8a04-baf7cb2aecb6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4136-528d-741c-9b8a-eea2ba016ae7" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_019e4136-528d-7631-8a04-baf7cb2aecb6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019fb3da-dc8a-7ed3-92d0-d0bbe320606d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4136-528d-741c-9b8a-eea2ba016ae7" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019fb3da-dc8a-7ed3-92d0-d0bbe320606d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019e4136-528d-757d-8595-380b7e46fd36" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4136-528d-741c-9b8a-eea2ba016ae7" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019e4136-528d-757d-8595-380b7e46fd36" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/StockBasedCompensationScheduleofRestrictedStockUnitsRSUsandPerformanceStockUnitsPSUsDetails" xlink:type="simple" xlink:href="tdup-20260630.xsd#StockBasedCompensationScheduleofRestrictedStockUnitsRSUsandPerformanceStockUnitsPSUsDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/StockBasedCompensationScheduleofRestrictedStockUnitsRSUsandPerformanceStockUnitsPSUsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4136-528d-7b58-8d11-bb9518f51c8a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4136-528d-71ba-a978-d241aead3db3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4136-528d-7b58-8d11-bb9518f51c8a" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4136-528d-71ba-a978-d241aead3db3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e4136-528d-7bda-8571-cc51c4f8a4ee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4136-528d-71ba-a978-d241aead3db3" xlink:to="loc_us-gaap_AwardTypeAxis_019e4136-528d-7bda-8571-cc51c4f8a4ee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4136-528d-7bda-8571-cc51c4f8a4ee_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e4136-528d-7bda-8571-cc51c4f8a4ee" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4136-528d-7bda-8571-cc51c4f8a4ee_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4136-528d-717f-b95e-2e4403cf49e2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e4136-528d-7bda-8571-cc51c4f8a4ee" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4136-528d-717f-b95e-2e4403cf49e2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_RestrictedStockUnitsRSUsAndPerformanceStockUnitsPSUsMember_019f6065-18d4-79bf-9377-88258d5d494d" xlink:href="tdup-20260630.xsd#tdup_RestrictedStockUnitsRSUsAndPerformanceStockUnitsPSUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4136-528d-717f-b95e-2e4403cf49e2" xlink:to="loc_tdup_RestrictedStockUnitsRSUsAndPerformanceStockUnitsPSUsMember_019f6065-18d4-79bf-9377-88258d5d494d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019f6079-cc44-718b-9c53-79fb431db71b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e4136-528d-717f-b95e-2e4403cf49e2" xlink:to="loc_us-gaap_PerformanceSharesMember_019f6079-cc44-718b-9c53-79fb431db71b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019f661c-1f44-7e9b-bec3-3a35abbc560c" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e4136-528d-71ba-a978-d241aead3db3" xlink:to="loc_srt_RangeAxis_019f661c-1f44-7e9b-bec3-3a35abbc560c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f661c-1f44-7e9b-bec3-3a35abbc560c_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019f661c-1f44-7e9b-bec3-3a35abbc560c" xlink:to="loc_srt_RangeMember_019f661c-1f44-7e9b-bec3-3a35abbc560c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f661c-1f44-7623-9543-452a43b0f5b0" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019f661c-1f44-7e9b-bec3-3a35abbc560c" xlink:to="loc_srt_RangeMember_019f661c-1f44-7623-9543-452a43b0f5b0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019f661c-3e52-7e9e-84cb-c74f02ed42cc" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f661c-1f44-7623-9543-452a43b0f5b0" xlink:to="loc_srt_MinimumMember_019f661c-3e52-7e9e-84cb-c74f02ed42cc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019f661c-6086-770b-8551-47d87a34919a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f661c-1f44-7623-9543-452a43b0f5b0" xlink:to="loc_srt_MaximumMember_019f661c-6086-770b-8551-47d87a34919a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e4136-528d-76b1-b715-8a4dbc08088e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4136-528d-7b58-8d11-bb9518f51c8a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e4136-528d-76b1-b715-8a4dbc08088e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e4136-528d-71fa-a8df-99b1a0a18f1a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e4136-528d-76b1-b715-8a4dbc08088e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e4136-528d-71fa-a8df-99b1a0a18f1a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e4136-528d-7905-8920-56b63659450e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e4136-528d-76b1-b715-8a4dbc08088e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e4136-528d-7905-8920-56b63659450e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019e4136-528d-729d-8b20-31f2e844ceb6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e4136-528d-76b1-b715-8a4dbc08088e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019e4136-528d-729d-8b20-31f2e844ceb6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019e4136-528d-7d84-8c26-15ae1799a0bb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e4136-528d-76b1-b715-8a4dbc08088e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019e4136-528d-7d84-8c26-15ae1799a0bb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e4136-528d-7bcd-93fd-3ae1bb61e7e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e4136-528d-75bc-b4b1-e894681bc41a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4136-528d-7b58-8d11-bb9518f51c8a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e4136-528d-75bc-b4b1-e894681bc41a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e4136-528d-710f-82d3-f27af7e71e27" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e4136-528d-75bc-b4b1-e894681bc41a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e4136-528d-710f-82d3-f27af7e71e27" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e4136-528d-7c37-8096-aecdf310b558" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e4136-528d-75bc-b4b1-e894681bc41a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e4136-528d-7c37-8096-aecdf310b558" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019e4136-528d-7a3d-9cde-a6b068e17e31" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e4136-528d-75bc-b4b1-e894681bc41a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019e4136-528d-7a3d-9cde-a6b068e17e31" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019e4136-528d-7308-b18a-25b1ada7fd77" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e4136-528d-75bc-b4b1-e894681bc41a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019e4136-528d-7308-b18a-25b1ada7fd77" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e4136-528d-7920-8b6d-c1671403487c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesEarnedAsAPercentOfTarget_019fb405-e430-789f-be28-ea0e6c8de0f4" xlink:href="tdup-20260630.xsd#tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesEarnedAsAPercentOfTarget"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e4136-528d-7b58-8d11-bb9518f51c8a" xlink:to="loc_tdup_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesEarnedAsAPercentOfTarget_019fb405-e430-789f-be28-ea0e6c8de0f4" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://thredup.com/role/LossPerShareDetails" xlink:type="simple" xlink:href="tdup-20260630.xsd#LossPerShareDetails"/>
  <link:definitionLink xlink:role="http://thredup.com/role/LossPerShareDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e4136-528d-709c-90b3-bbb438387850" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019e4136-528d-783c-92e5-a267b3c75ba5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e4136-528d-709c-90b3-bbb438387850" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019e4136-528d-783c-92e5-a267b3c75ba5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e4136-528d-7983-93ae-e1c16ee33ef2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019e4136-528d-783c-92e5-a267b3c75ba5" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e4136-528d-7983-93ae-e1c16ee33ef2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e4136-528d-7983-93ae-e1c16ee33ef2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e4136-528d-7983-93ae-e1c16ee33ef2" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e4136-528d-7983-93ae-e1c16ee33ef2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e4136-528d-7f16-9aa0-c700a3063afe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e4136-528d-7983-93ae-e1c16ee33ef2" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e4136-528d-7f16-9aa0-c700a3063afe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e4136-528d-742b-abb4-86862d5a7e87" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e4136-528d-7f16-9aa0-c700a3063afe" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e4136-528d-742b-abb4-86862d5a7e87" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e4136-528d-7bba-b2ad-2ad3c1fd9456" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e4136-528d-7f16-9aa0-c700a3063afe" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e4136-528d-7bba-b2ad-2ad3c1fd9456" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_019e4136-528d-75dd-81a3-aa9ca92221eb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e4136-528d-7f16-9aa0-c700a3063afe" xlink:to="loc_us-gaap_EmployeeStockMember_019e4136-528d-75dd-81a3-aa9ca92221eb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019e4136-528d-7894-bdb6-ceab5718e215" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e4136-528d-709c-90b3-bbb438387850" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019e4136-528d-7894-bdb6-ceab5718e215" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#IndividualsOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ecd_AllIndividualsMember" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#ecd_AllIndividualsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_tdup_ChrisHomerMember_019e4136-51c6-763d-a9b3-233ea7957773" xlink:href="tdup-20260630.xsd#tdup_ChrisHomerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_tdup_ChrisHomerMember_019e4136-51c6-763d-a9b3-233ea7957773" xlink:type="arc" order="2"/>
  </link:definitionLink>
</link:linkbase>
