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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2022-06-22</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>2022-06-22</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>267937</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>133709</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnote.</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2022-06-22</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F3"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>79695</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>85514</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>26631</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>45757</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>34878</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>see footnote.</value>
                    <footnoteId id="F6"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">On June 22, 2022 Transportation Technology Ventures LLC  distributed, for no consideration, 267,937 shares of common stock of the Issuer 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 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>
        <footnote id="F3">Shares received from distribution on June 22,2022 as described in Footnote 1.</footnote>
        <footnote id="F4">Shares are held directly by Autonomous IOT Fund, L.P. (&quot;Motus-VGO&quot;). 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="F5">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="F6">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>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Jerry Allison (Attorney-in-Fact)</signatureName>
        <signatureDate>2022-06-23</signatureDate>
    </ownerSignature>
</ownershipDocument>
