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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2016-05-04</periodOfReport>

    <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>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>

    <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>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class B Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2016-05-04</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1250000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>515112</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F4"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Manager and Member</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2016-05-04</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1250000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1889071</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F4"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Manager and Member</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Class B Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>8354978</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <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>1951006</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <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>35675</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Class B Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>12658799</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F6"/>
                </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="F7"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Manager and Member</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Transaction represents the conversion of 1,250,000 shares of Class B common stock held by Cactus Holding Company II, LLC (&quot;Cactus II&quot;) into an equal number of shares of Class A common stock.  Pursuant to the Issuer's charter, shares of Class B common stock are convertible, at the option of the holders thereof and under certain other circumstances, on a one-for-one basis into shares of Class A common stock.  This transaction was previously reported by the reporting persons in an amendment to their Schedule 13D.</footnote>
        <footnote id="F2">Amount reflects a distribution of 5,054,978 shares of Class B Common Stock and 1,951,006 shares of Class A Common Stock from Cactus II to Cactus Holding Company III, LLC (&quot;Cactus III&quot;), and the contribution of such shares by Cactus III to Cactus Holding Company, LLC (&quot;Cactus I&quot;).  The reporting persons constitute all of the members of Cactus III, which is the sole member of Cactus II and Cactus I.  Accordingly, these transactions represent a change in form of indirect beneficial ownership and did not affect the pecuniary interest of the reporting persons in these shares.</footnote>
        <footnote id="F3">Amount reflects a distribution by M Capital Group Investors, LLC (&quot;M Capital I&quot;) of an aggregate of 2,000,000 shares of Class B common stock to certain of its members who are not reporting persons.  This distribution did not change the number of shares in which the reporting persons have a pecuniary interest.</footnote>
        <footnote id="F4">Shares are held by Cactus II, an entity for which the Jerry and Vickie Moyes Family Trust (the &quot;JVMF Trust&quot;) serves as the sole manager.  Cactus II is wholly owned by Cactus III, of which the reporting persons are all of the members.</footnote>
        <footnote id="F5">Shares are held by Cactus I, an entity for which the JVMF Trust serves as the sole manager.  Cactus I is wholly owned by Cactus III, of which the reporting persons are all of the members.</footnote>
        <footnote id="F6">Shares are held by M Capital I, an entity for which the JVMF Trust serves as the sole manager.  Cactus I is one of the members of M Capital I.  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="F7">Shares are held by M Capital Group Investors II, LLC (&quot;M Capital II&quot;), an entity for which the JVMF Trust serves as the sole manager.  The reporting persons 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>2016-06-08</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Vickie Moyes, by Earl Scudder, attorney-in-fact, pursuant to a POA previously filed</signatureName>
        <signatureDate>2016-06-08</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>2016-06-08</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>2016-06-08</signatureDate>
    </ownerSignature>
</ownershipDocument>
