<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2017-12-22</periodOfReport>

    <notSubjectToSection16>true</notSubjectToSection16>

    <issuer>
        <issuerCik>0001592058</issuerCik>
        <issuerName>Party City Holdco Inc.</issuerName>
        <issuerTradingSymbol>PRTY</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001034196</rptOwnerCik>
            <rptOwnerName>ADVENT INTERNATIONAL CORP/MA</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>800 BOYLSTON STREET</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>BOSTON</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02199</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isTenPercentOwner>true</isTenPercentOwner>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001430919</rptOwnerCik>
            <rptOwnerName>ADVENT INTERNATIONAL GPE VI LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ADVENT INTERNATIONAL CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>800 BOYLSTON STREET</rptOwnerStreet2>
            <rptOwnerCity>BOSTON</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02199</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isTenPercentOwner>true</isTenPercentOwner>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001430920</rptOwnerCik>
            <rptOwnerName>ADVENT INTERNATIONAL GPE VI-A LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ADVENT INTERNATIONAL CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>800 BOYLSTON STREET</rptOwnerStreet2>
            <rptOwnerCity>BOSTON</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02199</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isTenPercentOwner>true</isTenPercentOwner>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001430926</rptOwnerCik>
            <rptOwnerName>ADVENT INTERNATIONAL GPE VI-B LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ADVENT INTERNATIONAL CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>800 BOYLSTON STREET</rptOwnerStreet2>
            <rptOwnerCity>BOSTON</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02199</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isTenPercentOwner>true</isTenPercentOwner>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001430925</rptOwnerCik>
            <rptOwnerName>ADVENT INTERNATIONAL GPE VI-C LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ADVENT INTERNATIONAL CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>800 BOYLSTON STREET</rptOwnerStreet2>
            <rptOwnerCity>BOSTON</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02199</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isTenPercentOwner>true</isTenPercentOwner>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001430923</rptOwnerCik>
            <rptOwnerName>ADVENT INTERNATIONAL GPE VI-D LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ADVENT INTERNATIONAL CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>800 BOYLSTON STREET</rptOwnerStreet2>
            <rptOwnerCity>BOSTON</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02199</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isTenPercentOwner>true</isTenPercentOwner>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001430921</rptOwnerCik>
            <rptOwnerName>ADVENT INTERNATIONAL GPE VI-E LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ADVENT INTERNATIONAL CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>800 BOYLSTON STREET</rptOwnerStreet2>
            <rptOwnerCity>BOSTON</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02199</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isTenPercentOwner>true</isTenPercentOwner>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001430922</rptOwnerCik>
            <rptOwnerName>ADVENT INTERNATIONAL GPE VI-F LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ADVENT INTERNATIONAL CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>800 BOYLSTON STREET</rptOwnerStreet2>
            <rptOwnerCity>BOSTON</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02199</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isTenPercentOwner>true</isTenPercentOwner>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001432552</rptOwnerCik>
            <rptOwnerName>ADVENT INTERNATIONAL GPE VI-G LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ADVENT INTERNATIONAL CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>800 BOYLSTON STREET</rptOwnerStreet2>
            <rptOwnerCity>BOSTON</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02199</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isTenPercentOwner>true</isTenPercentOwner>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2017-12-22</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>19841694</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>12.2</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Notes</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                    <footnoteId id="F5"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">The shares of common stock of the Issuer were sold in connection with a stock purchase agreement by and between the Issuer and the Advent-Party City Acquisition Limited Partnership (&quot;ACPA&quot;) pursuant to which the Issuer repurchased the shares.  Following the repurchase, ACPA no longer holds any shares of the Issuer.</footnote>
        <footnote id="F2">APCA directly held the shares of common stock of the Issuer prior to the transaction being reported. The Advent Funds (as defined below) directly own all of the partnership interests in APCA. APCA, together with Advent International Corporation (&quot;AIC&quot;), Advent International LLC (&quot;AILLC&quot;), GPE VI GP Limited Partnership (&quot;GPE VI&quot;), GPE VI GP (Delaware) Limited Partnership (&quot;GPE VI D&quot;) and the Advent Funds, are referred to herein as the &quot;Reporting Persons&quot;.</footnote>
        <footnote id="F3">AIC is the manager of AILLC, which is the general partner of each of GPE VI, GPE VI D, Advent Partners GPE VI 2008 Limited Partnership (&quot;GPE 2008), Advent Partners GPE VI 2009 Limited Partnership (&quot;GPE 2009&quot;), Advent Partners GPE VI 2010 Limited Partnership (&quot;GPE 2010&quot;), Advent Partners GPE VI-A Limited Partnership (&quot;GPE A&quot;), and Advent Partners GPE VI-A 2010 Limited Partnership (&quot;GPE A 2010&quot;, together with GPE 2008, GPE 2009, GPE 2010 and GPE A, collectively, the &quot;Advent Partner Entities&quot;).</footnote>
        <footnote id="F4">GPE VI D is the general partner of the following entities: Advent International GPE VI-C Limited Partnership, Advent International GPE VI-D Limited Partnership and GPE VI-E Limited Partnership (collectively, the &quot;GPE VI D Funds&quot;).</footnote>
        <footnote id="F5">GPE VI is the general partner of the following entities: Advent International GPE VI-G Limited Partnership, Advent International GPE VI Limited Partnership, Advent International GPE VI-A Limited Partnership, Advent Internal GPE VI-B Limited Partnership, and Advent International GPE VI-F Limited Partnership (collectively, the &quot;GPE VI Funds&quot; and together with the GPE VI D Funds and the Advent Partner Entities, collectively the &quot;Advent Funds&quot;).</footnote>
    </footnotes>

    <remarks>This Form 4 is the first of two Forms 4 being filed relating to the same event. The Form 4 has been split into two filings because there are more than 10 Reporting Persons in total, and the SEC's EDGAR filing system limits a single Form 4 to a maximum of 10 Reporting Persons. Each Form 4 will be filed by Designated Filer Advent International Corporation.</remarks>

    <ownerSignature>
        <signatureName>/s/ Richard Terranova, Attorney-in-Fact</signatureName>
        <signatureDate>2017-12-27</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Richard Terranova, Attorney-in-Fact</signatureName>
        <signatureDate>2017-12-27</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Richard Terranova, Attorney-in-Fact</signatureName>
        <signatureDate>2017-12-27</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Richard Terranova, Attorney-in-Fact</signatureName>
        <signatureDate>2017-12-27</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Richard Terranova, Attorney-in-Fact</signatureName>
        <signatureDate>2017-12-27</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Richard Terranova, Attorney-in-Fact</signatureName>
        <signatureDate>2017-12-27</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Richard Terranova, Attorney-in-Fact</signatureName>
        <signatureDate>2017-12-27</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Richard Terranova, Attorney-in-Fact</signatureName>
        <signatureDate>2017-12-27</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Richard Terranova, Attorney-in-Fact</signatureName>
        <signatureDate>2017-12-27</signatureDate>
    </ownerSignature>
</ownershipDocument>
