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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2017-04-19</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000887919</issuerCik>
        <issuerName>PREMIER FINANCIAL BANCORP INC</issuerName>
        <issuerTradingSymbol>PFBI</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001157292</rptOwnerCik>
            <rptOwnerName>WALKER ROBERT W</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O PREMIER FINANCIAL BANCORP, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>2883 5TH AVENUE</rptOwnerStreet2>
            <rptOwnerCity>HUNTINGTON</rptOwnerCity>
            <rptOwnerState>WV</rptOwnerState>
            <rptOwnerZipCode>25702</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chief Executive Officer</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>PFBI Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2017-04-19</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>6000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>86548</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>PFBI Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>6261</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By 401(k) Plan</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>PFBI Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>10349</value>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Spouse</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Based upon a plan statement dated December 31, 2016, the reporting person owned 6,261 shares of PFBI Common Stock under the PFBI 401(k) plan.</footnote>
        <footnote id="F2">On November 16, 2016, issuer declared a 10% stock dividend. As a result, on December 9, 2016, Premier Financial Bancorp, Inc. issued its shareholders 1 share of common stock for each 10 shares of common stock already owned as of a December 2, 2016 record date. Mr. Walker's direct ownership increased by 7,321 shares prior to the reported transaction as result of this 10% stock dividend.</footnote>
        <footnote id="F3">On November 16, 2016, issuer declared a 10% stock dividend. As a result, on December 9, 2016, Premier Financial Bancorp, Inc. issued its shareholders 1 share of common stock for each 10 shares of common stock already owned as of a December 2, 2016 record date. Mr. Walker's indirect ownership increased by 569 shares as result of this 10% stock dividend.</footnote>
        <footnote id="F4">On November 16, 2016, issuer declared a 10% stock dividend. As a result, on December 9, 2016, Premier Financial Bancorp, Inc. issued its shareholders 1 share of common stock for each 10 shares of common stock already owned as of a December 2, 2016 record date. Mr. Walker's indirect ownership increased by 940 shares as result of this 10% stock dividend.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Robert W. Walker</signatureName>
        <signatureDate>2017-04-21</signatureDate>
    </ownerSignature>
</ownershipDocument>
