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

    <schemaVersion>X0206</schemaVersion>

    <documentType>3/A</documentType>

    <periodOfReport>2013-10-07</periodOfReport>

    <dateOfOriginalSubmission>2013-10-17</dateOfOriginalSubmission>

    <noSecuritiesOwned>0</noSecuritiesOwned>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001507663</rptOwnerCik>
            <rptOwnerName>Moyes Michael</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>SWIFT TRANSPORTATION COMPANY</rptOwnerStreet1>
            <rptOwnerStreet2>PO BOX 1397</rptOwnerStreet2>
            <rptOwnerCity>TOLLESON</rptOwnerCity>
            <rptOwnerState>AZ</rptOwnerState>
            <rptOwnerZipCode>85353</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle></officerTitle>
            <otherText>Member of 10% owner group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Class B Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>758800</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F1"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>As Trustee</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Class B Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>758800</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F2"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>As Trustee</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Class B Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>758800</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F3"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>As Trustee</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Class B Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>758800</value>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F4"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>As Trustee</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Class B Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>758800</value>
                    <footnoteId id="F5"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F5"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>As Trustee</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Class B Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>758800</value>
                    <footnoteId id="F6"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F6"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Trust</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Class B Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>17958799</value>
                    <footnoteId id="F7"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F7"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Member</value>
                    <footnoteId id="F7"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Shares held by the Todd Moyes Trust, over which the reporting person is the trustee. The number of shares reported amends the number of shares previously reported on the reporting person's Form 3, filed with the Securities and Exchange Commission (&quot;SEC&quot;) on October 17, 2013, to reflect the transfer of 68,509 shares from M Capital Group Investors, LLC (&quot;M Capital&quot;) to the Todd Moyes Trust on October 4, 2013.  The reporting person disclaims beneficial ownership of these securities except to the extent of his pecuniary interest therein and the inclusion of these shares in this report shall not be deemed an admission of beneficial ownership of these shares for Section 16 or for any other purpose.</footnote>
        <footnote id="F2">Shares held by the Hollie Moyes Trust, over which the reporting person is the trustee.  The number of shares reported amends the number of shares previously reported on the reporting person's Form 3, filed with the SEC on October 17, 2013, to reflect the transfer of 68,509 shares from M Capital to the Hollie Moyes Trust on October 4, 2013.  The reporting person disclaims beneficial ownership of these securities except to the extent of his pecuniary interest therein and the inclusion of these shares in this report shall not be deemed an admission of beneficial ownership of these shares for Section 16 or for any other purpose.</footnote>
        <footnote id="F3">Shares held by the Chris Moyes Trust, over which the reporting person is the trustee. The number of shares reported amends the number of shares previously reported on the reporting person's Form 3, filed with the SEC on October 17, 2013, to reflect the transfer of 68,509 shares from M Capital to the Chris Moyes Trust on October 4, 2013.  The reporting person disclaims beneficial ownership of these securities except to the extent of his pecuniary interest therein and the inclusion of these shares in this report shall not be deemed an admission of beneficial ownership of these shares for Section 16 or for any other purpose.</footnote>
        <footnote id="F4">Shares held by the Lyndee Moyes Nester Trust, over which the reporting person is the trustee.  The number of shares reported amends the number of shares previously reported on the reporting person's Form 3, filed with the SEC on October 17, 2013, to reflect the transfer of 68,509 shares from M Capital to the Lyndee Moyes Nester Trust on October 4, 2013.  The reporting person disclaims beneficial ownership of these securities except to the extent of his pecuniary interest therein and the inclusion of these shares in this report shall not be deemed an admission of beneficial ownership of these shares for Section 16 or for any other purpose.</footnote>
        <footnote id="F5">Shares held by the Marti Lyn Moyes Trust, over which the reporting person is the trustee.  The number of shares reported amends the number of shares previously reported on the reporting person's Form 3, filed with the SEC on October 17, 2013, to reflect the transfer of 68,509 shares from M Capital to the Marti Lyn Moyes Trust on October 4, 2013.  The reporting person disclaims beneficial ownership of these securities except to the extent of his pecuniary interest therein and the inclusion of these shares in this report shall not be deemed an admission of beneficial ownership of these shares for Section 16 or for any other purpose.</footnote>
        <footnote id="F6">Shares held by the Michael J. Moyes Trust, of which the reporting person is the beneficiary.  The number of shares reported amends the number of shares previously reported on the reporting person's Form 3, filed with the SEC on October 17, 2013, to reflect the transfer of 68,509 shares from M Capital to the Michael J. Moyes Trust on October 4, 2013.  The reporting person disclaims beneficial ownership of these securities except to the extent of his pecuniary interest therein and the inclusion of these shares in this report shall not be deemed an admission of beneficial ownership of these shares for Section 16 or for any other purpose.</footnote>
        <footnote id="F7">Shares held by M Capital Group Investors, LLC, of which the Todd Moyes Trust, the Hollie Moyes Trust, the Chris Moyes Trust, the Lyndee Moyes Nester Trust, the Marti Lyn Moyes Trust, and the Michael J. Moyes Trust (collectively, the &quot;Trusts&quot;) are certain of the members.  The reporting person is the trustee of each of the Trusts with the exception of the Michael J. Moyes Trust, of which the reporting person is the beneficiary.  The number of shares reported amends the number of shares previously reported on the reporting person's Form 3, filed with the SEC on October 17, 2013, to reflect the disposition of 2,366,685 shares by M Capital on October 4, 2013.  The reporting person disclaims beneficial ownership of these securities except to the extent of his pecuniary interest therein and the inclusion of these shares in this report shall not be deemed an admission of beneficial ownership of these shares for Section 16 or for any other purpose.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Michael Moyes, by Earl Scudder, attorney-in-fact, pursuant to a POA previously filed</signatureName>
        <signatureDate>2013-10-31</signatureDate>
    </ownerSignature>
</ownershipDocument>
