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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2016-02-26</periodOfReport>

    <dateOfOriginalSubmission>2016-03-01</dateOfOriginalSubmission>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001492691</issuerCik>
        <issuerName>SWIFT TRANSPORTATION Co</issuerName>
        <issuerTradingSymbol>SWFT</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0000901736</rptOwnerCik>
            <rptOwnerName>MOYES JERRY</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>SWIFT TRANSPORTATION COMPANY</rptOwnerStreet1>
            <rptOwnerStreet2>2200 SOUTH 75TH AVENUE</rptOwnerStreet2>
            <rptOwnerCity>PHOENIX</rptOwnerCity>
            <rptOwnerState>AZ</rptOwnerState>
            <rptOwnerZipCode>85043</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle>CEO</officerTitle>
            <otherText>Member of 10% group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001272829</rptOwnerCik>
            <rptOwnerName>MOYES VICKIE</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>P.O. BOX 1397</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>TOLLESON</rptOwnerCity>
            <rptOwnerState>AZ</rptOwnerState>
            <rptOwnerZipCode>85353</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001092565</rptOwnerCik>
            <rptOwnerName>MOYES JERRY &amp; VICKIE FAMILY TRUST</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O SWIFT AVIATION GROUP, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>2710 E. OLD TOWER ROAD</rptOwnerStreet2>
            <rptOwnerCity>PHOENIX</rptOwnerCity>
            <rptOwnerState>AZ</rptOwnerState>
            <rptOwnerZipCode>85034</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2016-02-26</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>17194</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>17.96</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>35675</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Class B Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>14658799</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Manager</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Class B Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3300000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F4"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Manager and Member</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Class B Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>6820090</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F5"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Manager and Member</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Class A Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2590077</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F5"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Manager and Member</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Class B Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>26213049</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F6"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Manager and Member</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Represents the number of shares deemed withheld to satisfy tax withholding obligations upon the vesting of performance shares granted to the reporting person pursuant to the Company's 2007 Omnibus Incentive Plan.  The reporting person previously reported a sale of 17,300 shares to cover taxes associated with the vesting.  That sale was canceled prior to settlement and the reporting person did not dispose of any shares or receive any proceeds in connection with the sale.  This Form 4 is being amended to reflect the cancellation of the sale and the withholding of shares to satisfy tax withholding obligations in lieu of selling shares.</footnote>
        <footnote id="F2">This Form 4 is also being amended to reflect the inclusion of these shares and additional reporting persons.</footnote>
        <footnote id="F3">Shares are held by M Capital Group Investors, LLC (&quot;M Capital I&quot;), and indirectly by the Jerry and Vickie Moyes Family Trust (the &quot;JVMF Trust&quot;), as the sole manager of M Capital I. Mr. and Mrs. Moyes serve as co-trustees of the JVMF Trust. The reporting persons disclaim any beneficial ownership of these securities except to the extent of their pecuniary interest therein, and the inclusion of these securities in this report shall not be deemed an admission of beneficial ownership of these securities for Section 16 or for any other purpose.</footnote>
        <footnote id="F4">Shares are held by Cactus Holding Company, LLC (&quot;Cactus I&quot;), an entity for which the JVMF Trust serves as sole manager.  The JVMF Trust, Mr. Moyes, and Mrs. Moyes are all of the members of Cactus I.</footnote>
        <footnote id="F5">Shares are held by Cactus Holding Company II, LLC (&quot;Cactus II&quot;), an entity for which the JVMF Trust serves as sole manager.  The JVMF Trust, Mr. Moyes, and Mrs. Moyes are all of the members of Cactus II.</footnote>
        <footnote id="F6">Shares are held by M Capital Group Investors II, LLC (&quot;M Capital II&quot;), and indirectly by the JVMF Trust, as the sole manager of M Capital II.  Mr. Moyes, Mrs. Moyes, and the JVMF Trust constitute certain of the members of M Capital II.  The reporting persons disclaim any beneficial ownership of these securities except to the extent of their pecuniary interest therein, and the inclusion of these securities in this report shall not be deemed an admission of beneficial ownership of these securities for Section 16 or for any other purpose.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Jerry Moyes, by Earl Scudder, attorney-in-fact, pursuant to a POA previously filed</signatureName>
        <signatureDate>2016-03-25</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Vickie Moyes, by Earl Scudder, attorney-in-fact, pursuant to a POA previously filed</signatureName>
        <signatureDate>2016-03-25</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Jerry Moyes, Co-Trustee of the Jerry and Vickie Moyes Family Trust, by Earl Scudder, attorney-in-fact, pursuant to a POA previously filed</signatureName>
        <signatureDate>2016-03-25</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Vickie Moyes, Co-Trustee of the Jerry and Vickie Moyes Family Trust, by Earl Scudder, attorney-in-fact, pursuant to a POA previously filed</signatureName>
        <signatureDate>2016-03-25</signatureDate>
    </ownerSignature>
</ownershipDocument>
