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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2022-04-27</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001794621</issuerCik>
        <issuerName>Quanergy Systems, Inc.</issuerName>
        <issuerTradingSymbol>QNGY</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001908622</rptOwnerCik>
            <rptOwnerName>DiSanto Jim</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O QUANERGY SYSTEMS, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>433 LAKESIDE DRIVE</rptOwnerStreet2>
            <rptOwnerCity>SUNNYVALE</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94085</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2022-04-27</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>60996</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>26631</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnote.</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>45757</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnote.</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>34878</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnote.</value>
                    <footnoteId id="F4"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>401646</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnote.</value>
                    <footnoteId id="F5"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5819</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">On April 27, 2022, Motus-VGO Autonomous IOT Fund, L.P. (&quot;Motus-VGO&quot;)  distributed, for no consideration, 60,996 shares of common stock of the Issuer (the Motus-VGO Shares) to its limited partners, representing such partner's pro rata interests in such shares. The aforementioned distribution was made in accordance with the exemption afforded by Rules 16a-13 and 16a-9 of the Securities Exchange Act of 1934, as amended.</footnote>
        <footnote id="F2">Shares are held directly by Motus-VGO.  Motus-VGO is managed by Motus-VGO GP LLC and Motus-VGO GP LLC is controlled by Mr. DiSanto. Accordingly, Mr. DiSanto may be deemed to be a beneficial owner of the shares held by Motus-VGO.</footnote>
        <footnote id="F3">Shares are held directly by Transportation Technology Ventures II, L.P. (&quot;TTV II, L.P.&quot;)TTV II, L.P. is managed by Transportation Technology Ventures LLC and Transportation Technology Ventures LLC is controlled by Mr. DiSanto. Accordingly, Mr. DiSanto may be deemed to be a beneficial owner of the shares held by TTV II, L.P.</footnote>
        <footnote id="F4">Shares are held directly by Transportation Technology Ventures V, L.P. (&quot;TTV V, L.P.&quot;) TTV V, L. P. is managed by Transportation Technology Ventures LLC and Transportation Technology Ventures LLC is controlled by Mr. DiSanto. Accordingly, Mr. DiSanto may be deemed to be a beneficial owner of the shares held by TTV V, L.P.</footnote>
        <footnote id="F5">Shares are directly held by Transportation Technology Ventures LLC and Transportation Technology Ventures LLC is controlled by Mr. DiSanto.  Accordingly, Mr. DiSanto may be deemed to be a beneficial owner of the shares held by  Transportation Technology Ventures LLC.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Jerry Allison (Attorney-in-Fact)</signatureName>
        <signatureDate>2022-04-28</signatureDate>
    </ownerSignature>
</ownershipDocument>
