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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2015-05-01</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000106455</issuerCik>
        <issuerName>WESTMORELAND COAL Co</issuerName>
        <issuerTradingSymbol>WLB</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001552546</rptOwnerCik>
            <rptOwnerName>Werner Russell H</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>9540 SOUTH MAROON CIRCLE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 200</rptOwnerStreet2>
            <rptOwnerCity>ENGLEWOOD</rptOwnerCity>
            <rptOwnerState>CO</rptOwnerState>
            <rptOwnerZipCode>80112</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Corporate Controller &amp; PAO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2015-05-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>5572</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>28.47</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4756</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>by 401(k) Plan</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">The reporting person undertakes to provide to Westmoreland Coal Company, any security holder of Westmoreland Coal Company, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in Footnote 2 to this Form 4.</footnote>
        <footnote id="F2">The average share price for the 5,572 shares sold was $28.4669, with the lowest per share sale price being $28.425 and the highest per share sale price being $28.49.</footnote>
        <footnote id="F3">The reporting person acquired additional shares of common stock under the Westmoreland Coal Company 401(k) plan since the date of the last report.  The information in this report is based on plan information as of May 4, 2015.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Jennifer S. Grafton by Power of Attorney</signatureName>
        <signatureDate>2015-05-04</signatureDate>
    </ownerSignature>
</ownershipDocument>
[{"type": "4", "sequence": "1", "filename": "wf-form4_143078021461337.xml", "description": "FORM 4", "secsgml_size_bytes": 4719}]
X0306
4
2015-05-01
0
0000106455
WESTMORELAND COAL Co
WLB
0001552546
Werner Russell H
9540 SOUTH MAROON CIRCLE
SUITE 200
ENGLEWOOD
CO
80112
0
1
0
0
Corporate Controller & PAO
Common Stock
2015-05-01
4
S
0
5572
28.47
D
0
D
Common Stock
4756
I
by 401(k) Plan
The reporting person undertakes to provide to Westmoreland Coal Company, any security holder of Westmoreland Coal Company, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in Footnote 2 to this Form 4.
The average share price for the 5,572 shares sold was $28.4669, with the lowest per share sale price being $28.425 and the highest per share sale price being $28.49.
The reporting person acquired additional shares of common stock under the Westmoreland Coal Company 401(k) plan since the date of the last report.  The information in this report is based on plan information as of May 4, 2015.
/s/ Jennifer S. Grafton by Power of Attorney
2015-05-04