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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2015-02-27</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000101199</issuerCik>
        <issuerName>UNITED FIRE GROUP INC</issuerName>
        <issuerTradingSymbol>UFCS</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001550220</rptOwnerCik>
            <rptOwnerName>Carlton Scott L</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>118 SECOND AVENUE SE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>CEDAR RAPIDS</rptOwnerCity>
            <rptOwnerState>IA</rptOwnerState>
            <rptOwnerZipCode>52401</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2015-02-27</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>G</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value>E</value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>518</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>92933</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2015-03-04</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>G</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value>E</value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>880</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>92053</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">The number of shares held directly by the Reporting Person  includes: 91,647 shares held directly by the Reporting Person and 1,286 shares of restricted stock issued under the Issuer's Non-Qualified Non-Employee Director Stock Option and Restricted Stock Plan which vest, subject to certain conditions, on May 21, 2017.</footnote>
        <footnote id="F2">The number of shares held directly by the Reporting Person  includes: 90,767 shares held directly by the Reporting Person and 1,286 shares of restricted stock issued under the Issuer's Non-Qualified Non-Employee Director Stock Option and Restricted Stock Plan which vest, subject to certain conditions, on May 21, 2017.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Scott L. Carlton by Barrie W. Ernst, Attorney-in-Fact</signatureName>
        <signatureDate>2015-03-04</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2015-02-27
0
0000101199
UNITED FIRE GROUP INC
UFCS
0001550220
Carlton Scott L
118 SECOND AVENUE SE
CEDAR RAPIDS
IA
52401
1
0
0
0
Common Stock
2015-02-27
5
G
0
E
518
0
D
92933
D
Common Stock
2015-03-04
5
G
0
E
880
0
D
92053
D
The number of shares held directly by the Reporting Person  includes: 91,647 shares held directly by the Reporting Person and 1,286 shares of restricted stock issued under the Issuer's Non-Qualified Non-Employee Director Stock Option and Restricted Stock Plan which vest, subject to certain conditions, on May 21, 2017.
The number of shares held directly by the Reporting Person  includes: 90,767 shares held directly by the Reporting Person and 1,286 shares of restricted stock issued under the Issuer's Non-Qualified Non-Employee Director Stock Option and Restricted Stock Plan which vest, subject to certain conditions, on May 21, 2017.
/s/ Scott L. Carlton by Barrie W. Ernst, Attorney-in-Fact
2015-03-04