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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2014-08-22</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000095953</issuerCik>
        <issuerName>SYNALLOY CORP</issuerName>
        <issuerTradingSymbol>SYNL</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001184740</rptOwnerCik>
            <rptOwnerName>Wright Murray H</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>DURRETTECRUMP LLC</rptOwnerStreet1>
            <rptOwnerStreet2>1111 EAST MAIN STREET, 16TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>RICHMOND</rptOwnerCity>
            <rptOwnerState>VA</rptOwnerState>
            <rptOwnerZipCode>23219</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>579</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Spousal IRA</value>
                    <footnoteId id="F1"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>35580</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>IRA</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1103</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>131639</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Revocable Trust</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4251</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Spouse</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Trust for Minor Son</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>20000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Limited Family Partnership</value>
                    <footnoteId id="F4"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5630</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Trust for Minor Daughter</value>
                    <footnoteId id="F5"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">The reporting person expressly disclaims beneficial ownership of these securities and filing this report shall not be construed as an admission of reporting person's ownership thereof for purposes of Section 16 of the Securities Act, or otherwise.</footnote>
        <footnote id="F2">The reporting person expressly disclaims beneficial ownership of these securities and filing this report shall not be construed as an admission of my benefitial ownership thereof for purposes of Section 16 of the Securities, or otherwise.</footnote>
        <footnote id="F3">The reporting person no longer has a reportable beneficial interest in 6260 shares of Synalloy common stock owned by his son and included in the reporting person's prior ownership reports.</footnote>
        <footnote id="F4">Rivendell Estates is a Family Partnership of which Mr. Wright is General Partner.</footnote>
        <footnote id="F5">The reporting person disclaims beneficial ownership of this minor child's shares for purposes of Section 16 of the Securities Act, or for any other purpose.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Cheryl C. Carter, Power of Attorney for Murray H. Wright</signatureName>
        <signatureDate>2014-08-26</signatureDate>
    </ownerSignature>
</ownershipDocument>
[{"type": "4", "sequence": "1", "filename": "wf-form4_140905704668572.xml", "description": "FORM 4", "secsgml_size_bytes": 8460}]
X0306
4
2014-08-22
0
0000095953
SYNALLOY CORP
SYNL
0001184740
Wright Murray H
DURRETTECRUMP LLC
1111 EAST MAIN STREET, 16TH FLOOR
RICHMOND
VA
23219
1
0
0
0
Common Stock
579
I
Spousal IRA
Common Stock
35580
I
IRA
Common Stock
1103
D
Common Stock
131639
I
Revocable Trust
Common Stock
4251
I
Spouse
Common Stock
0
I
Trust for Minor Son
Common Stock
20000
I
Limited Family Partnership
Common Stock
5630
I
Trust for Minor Daughter
The reporting person expressly disclaims beneficial ownership of these securities and filing this report shall not be construed as an admission of reporting person's ownership thereof for purposes of Section 16 of the Securities Act, or otherwise.
The reporting person expressly disclaims beneficial ownership of these securities and filing this report shall not be construed as an admission of my benefitial ownership thereof for purposes of Section 16 of the Securities, or otherwise.
The reporting person no longer has a reportable beneficial interest in 6260 shares of Synalloy common stock owned by his son and included in the reporting person's prior ownership reports.
Rivendell Estates is a Family Partnership of which Mr. Wright is General Partner.
The reporting person disclaims beneficial ownership of this minor child's shares for purposes of Section 16 of the Securities Act, or for any other purpose.
Cheryl C. Carter, Power of Attorney for Murray H. Wright
2014-08-26