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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2015-02-18</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>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></nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Forward Sale Contract (obligation to sell)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
                <footnoteId id="F3"/>
                <footnoteId id="F4"/>
                <footnoteId id="F5"/>
                <footnoteId id="F6"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2015-02-18</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
                <footnoteId id="F3"/>
                <footnoteId id="F4"/>
                <footnoteId id="F5"/>
                <footnoteId id="F6"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                    <footnoteId id="F5"/>
                    <footnoteId id="F6"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
                <footnoteId id="F3"/>
                <footnoteId id="F4"/>
                <footnoteId id="F5"/>
                <footnoteId id="F6"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
                <footnoteId id="F3"/>
                <footnoteId id="F4"/>
                <footnoteId id="F5"/>
                <footnoteId id="F6"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>25994016</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F9"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Manager and Member</value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Forward Sale Contract (obligation to sell)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
                <footnoteId id="F3"/>
                <footnoteId id="F4"/>
                <footnoteId id="F5"/>
                <footnoteId id="F6"/>
                <footnoteId id="F7"/>
                <footnoteId id="F8"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2015-02-18</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
                <footnoteId id="F3"/>
                <footnoteId id="F4"/>
                <footnoteId id="F5"/>
                <footnoteId id="F6"/>
                <footnoteId id="F7"/>
                <footnoteId id="F8"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                    <footnoteId id="F5"/>
                    <footnoteId id="F6"/>
                    <footnoteId id="F7"/>
                    <footnoteId id="F8"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
                <footnoteId id="F3"/>
                <footnoteId id="F4"/>
                <footnoteId id="F5"/>
                <footnoteId id="F6"/>
                <footnoteId id="F7"/>
                <footnoteId id="F8"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
                <footnoteId id="F3"/>
                <footnoteId id="F4"/>
                <footnoteId id="F5"/>
                <footnoteId id="F6"/>
                <footnoteId id="F7"/>
                <footnoteId id="F8"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>25994016</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F9"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Manager and Member</value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">On October 29, 2013, M Capital Group Investors II, LLC (&quot;M Capital II&quot;) entered into a Master Terms and Conditions for Prepaid Variable Share Forward Transactions (together with the supplemental confirmations thereto, the &quot;Forward Agreement&quot;) with Citibank, N.A. (&quot;Citibank&quot;). On February 18, 2015, M Capital II and Citibank amended the terms of the Forward Agreement by entering into amended and restated supplemental confirmations with respect thereto (together, the &quot;Amendment&quot;).  This Form 4 is being filed to report the Amendment.</footnote>
        <footnote id="F2">Under the Forward Agreement, M Capital II is obligated to deliver a variable amount of Class A Common Stock, par value $0.01 per share, of Swift Transportation Company (&quot;Class A Common Stock&quot;), or an equivalent amount of cash, upon certain dates specified in the Forward Agreement. The number of shares deliverable under the Forward Agreement depends on the &quot;Initial Reference Price,&quot; &quot;Forward Floor Price,&quot; &quot;Forward Cap Price,&quot; &quot;Number of Shares,&quot; &quot;Settlement Price,&quot; and, prior to the Amendment, the &quot;Cap Ratio,&quot; each as defined in the Forward Agreement. The Forward Agreement involves two tranches, each of which has its own economic terms. Prior to the Amendment, each tranche had twenty components.  Each component has its own &quot;Valuation Date&quot; and &quot;Number of Shares.&quot;</footnote>
        <footnote id="F3">Prior to the Amendment, under the first tranche the Initial Reference Price was defined as $22.54, the Forward Floor Price was defined as $22.54, the Forward Cap Price was defined as $28.175, the Cap Ratio was defined as 1.3, and the Number of Shares for each component was 685,000. Prior to the Amendment, under the second tranche the Initial Reference Price was defined as $22.54, the Forward Floor Price was defined as $20.286, the Forward Cap Price was defined as $36.064, the Cap Ratio was defined as 1.3, and the Number of Shares for each component was 614,700, with the exception of one component for which the Number of Shares was 614,716.</footnote>
        <footnote id="F4">The Settlement Price under both tranches is defined as the volume-weighted average price per share of Class A Common Stock on the New York Stock Exchange on the relevant &quot;Valuation Date&quot; (as defined in each component). Prior to the Amendment, the first tranche had twenty specified Valuation Dates ranging from January 4, 2016 through February 1, 2016. The second tranche has twenty specified Valuation Dates ranging from July 5, 2016 through August 1, 2016. The shares or cash deliverable by M Capital II must be delivered on the date that is one Settlement Cycle (as defined in the Forward Agreement) following the relevant Valuation Date.</footnote>
        <footnote id="F5">The maximum number of shares to be delivered under both tranches is 25,994,016 shares of Class A Common Stock, but the actual number of shares required to be delivered under each component depends on the Settlement Price with respect to such component. If the Settlement Price is less than or equal to the Forward Floor Price on the relevant Valuation Date, the Number of Shares attributable to that component must be delivered. If the Settlement Price is greater than the Forward Floor Price but less than or equal to the Forward Cap Price, the number of shares to be delivered equals the Forward Floor Price, divided by the Settlement Price, multiplied by the Number of Shares.</footnote>
        <footnote id="F6">Prior to the Amendment, if the Settlement Price was greater than the Forward Cap Price, the number of shares to be delivered was the sum of (1) the Forward Floor Price, divided by the Settlement Price, multiplied by the Number of Shares; and (2) (a) the excess of the Settlement Price over the Forward Cap Price, divided by the Settlement Price, multiplied by (b) the Number of Shares divided by the Cap Ratio.</footnote>
        <footnote id="F7">The Amendment amended the terms of the first tranche by (i) increasing the Forward Cap Price to $34.00; (ii) reducing the number of components to three and changing the Valuation Dates to be November 4, 5, and 6 of 2015, respectively; and (iii) changing the Number of Shares in each of the three components to 4,566,666, 4,566,666, and 4,566,668, respectively.  The Amendment amended the terms of the second tranche by (i) increasing the Forward Floor Price to $23.30; and (ii) increasing the Forward Cap Price to $36.50.</footnote>
        <footnote id="F8">The Amendment amended both tranches to change the number of shares to be delivered if the Settlement Price is greater than the Forward Cap Price to be equal to the product of (i) the Number of Shares and (ii) a fraction (a) the numerator of which is the sum of (x) the Forward Floor Price and (y) the Settlement Price minus the Forward Cap Price, and (b) the denominator of which is the Settlement Price.</footnote>
        <footnote id="F9">Shares are owned directly by M Capital II, and indirectly by the Jerry and Vickie Moyes Family Trust, as the sole manager of M Capital II. Jerry and Vickie Moyes are the sole trustees of the Jerry and Vickie Moyes Family Trust.  Jerry Moyes, Jerry and Vickie Moyes, jointly, and the Jerry and Vickie Moyes Family Trust are also 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 shares in this report shall not be deemed an admission of beneficial ownership of such shares for Section 16 or for any other purpose.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Jerry Moyes, by Earl Scudder, attorney-in-fact, pursuant to a POA previously filed</signatureName>
        <signatureDate>2015-02-20</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Vickie Moyes, by Earl Scudder, attorney-in-fact, pursuant to a POA previously filed</signatureName>
        <signatureDate>2015-02-20</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>2015-02-20</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>2015-02-20</signatureDate>
    </ownerSignature>
</ownershipDocument>
