<SUBMISSION>
<ACCESSION-NUMBER>0001179110-16-017005
<TYPE>4/A
<PUBLIC-DOCUMENT-COUNT>1
<PERIOD>20151231
<FILING-DATE>20160105
<DATE-OF-FILING-DATE-CHANGE>20160105
<ISSUER>
<COMPANY-DATA>
<CONFORMED-NAME>ATLANTIC POWER CORP
<CIK>0001419242
<ASSIGNED-SIC>4900
<IRS-NUMBER>550886410
<STATE-OF-INCORPORATION>A1
<FISCAL-YEAR-END>1231
</COMPANY-DATA>
<BUSINESS-ADDRESS>
<STREET1>3 ALLIED DRIVE
<STREET2>SUITE 220
<CITY>DEDHAM
<STATE>MA
<ZIP>02026
<PHONE>617-977-2400
</BUSINESS-ADDRESS>
<MAIL-ADDRESS>
<STREET1>3 ALLIED DRIVE
<STREET2>SUITE 220
<CITY>DEDHAM
<STATE>MA
<ZIP>02026
</MAIL-ADDRESS>
</ISSUER>
<REPORTING-OWNER>
<OWNER-DATA>
<CONFORMED-NAME>McNeil John Alexander
<CIK>0001489397
</OWNER-DATA>
<FILING-VALUES>
<FORM-TYPE>4/A
<ACT>34
<FILE-NUMBER>001-34691
<FILM-NUMBER>161321407
</FILING-VALUES>
<MAIL-ADDRESS>
<STREET1>200 CLARENDON STREET
<CITY>BOSTON
<STATE>MA
<ZIP>02116
</MAIL-ADDRESS>
</REPORTING-OWNER>
<DOCUMENT>
<TYPE>4/A
<SEQUENCE>1
<FILENAME>edgar.xml
<DESCRIPTION>FORM 4/A -
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2015-12-31</periodOfReport>

    <dateOfOriginalSubmission>2016-01-04</dateOfOriginalSubmission>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001419242</issuerCik>
        <issuerName>ATLANTIC POWER CORP</issuerName>
        <issuerTradingSymbol>AT</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001489397</rptOwnerCik>
            <rptOwnerName>McNeil John Alexander</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ATLANTIC POWER CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>3 ALLIED DRIVE, SUITE 220</rptOwnerStreet2>
            <rptOwnerCity>DEDHAM</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02026</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>5.60% Convertible Debentures due June 30, 2017</value>
                <footnoteId id="F9"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>13.08</value>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2015-12-31</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionTotalValue>
                    <value>72250</value>
                    <footnoteId id="F4"/>
                </transactionTotalValue>
                <transactionPricePerShare>
                    <value>64.30</value>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2015-12-31</value>
            </exerciseDate>
            <expirationDate>
                <value>2017-06-30</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common shares</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>5524</value>
                    <footnoteId id="F2"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <valueOwnedFollowingTransaction>
                    <value>72250</value>
                    <footnoteId id="F4"/>
                </valueOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>6.00% Convertible Debentures due December 31, 2019</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>10.48</value>
                <footnoteId id="F5"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2015-12-31</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionTotalValue>
                    <value>36125</value>
                    <footnoteId id="F8"/>
                </transactionTotalValue>
                <transactionPricePerShare>
                    <value>55.99</value>
                    <footnoteId id="F7"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2015-12-31</value>
            </exerciseDate>
            <expirationDate>
                <value>2019-12-31</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common shares</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>3448</value>
                    <footnoteId id="F6"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <valueOwnedFollowingTransaction>
                    <value>72250</value>
                    <footnoteId id="F10"/>
                    <footnoteId id="F8"/>
                </valueOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The 5.60% Convertible Debentures due June 30, 2017 are convertible into approximately 55.2486 common shares per Cdn$1,000 principal amount, at any time, at the option of the holder representing a conversion price of Cdn$18.10 per common share. The conversion price of Cdn$18.10 was converted to the U.S. dollar equivalent of $13.08 using the December 31, 2015 end of day exchange rate of .7225.</footnote>
        <footnote id="F2">The 5.60% Convertible Debentures due June 30, 2017 are convertible into approximately 55.2486 common shares per Cdn$1,000 principal amount, at any time, at the option of the holder representing a conversion price of Cdn$18.10 per common share. The Cdn$100,000 of convertible debentures converts to 5,524 common shares using the December 31, 2015 end of day exchange rate of .7225.</footnote>
        <footnote id="F3">The Cdn$100,000 principal of 5.60% Convertible Debentures due June 30, 2017 were purchased at a price of Cdn$89.00. This was converted to a U.S. dollar equivalent of $64.30 using the December 31, 2015 end of day exchange rate of .7225.</footnote>
        <footnote id="F4">The Cdn$100,000 principal of 5.60% Convertible Debentures due June 30, 2017 were converted to a U.S. dollar equivalent of $72,250 using the December 31, 2015 end of day exchange rate of .7225.</footnote>
        <footnote id="F5">The 6.00% Convertible Debentures due December 31, 2019 are convertible into approximately 68.9655 common shares per Cdn$1,000 principal amount, at any time, at the option of the holder representing a conversion price of Cdn$14.50 per common share. The conversion price of Cdn$14.50 was converted to the U.S. dollar equivalent of $10.48 using the December 31, 2015 end of day exchange rate of .7225.</footnote>
        <footnote id="F6">The 6.00% Convertible Debentures due December 31, 2019 are convertible into approximately 68.9655 common shares per Cdn$1,000 principal amount, at any time, at the option of the holder representing a conversion price of Cdn$14.50 per common share. The Cdn$50,000 of convertible debentures converts to 3,448 common shares using the December 31, 2015 end of day exchange rate of .7225.</footnote>
        <footnote id="F7">The Cdn$50,000 principal of 6.00% Convertible Debentures due December 31, 2019 were purchased at a price of Cdn$77.50. This was converted to a U.S. dollar equivalent of $55.99 using the December 31, 2015 end of day exchange rate of .7225.</footnote>
        <footnote id="F8">The Cdn$50,000 principal of 6.00% Convertible Debentures due December 31, 2019 were converted to a U.S. dollar equivalent of $36,125 using the December 31, 2015 end of day exchange rate of .7225.</footnote>
        <footnote id="F9">The Form 4 filed for Mr. McNeil on January 4, 2016 incorrectly identified the instrument purchased as 6.25% Convertible Debentures due March 15, 2017. The correct title of the derivative security in Column 1 of Table II should have been 5.60% Convertible Debenture due June 30, 2017. All pertinent information has been corrected based on the investment actually purchased. The Number of Derivative Securities Beneficially Owned Following Reported Transaction in Column 9 of Table II has also been corrected to $72,250 from $151,725.</footnote>
        <footnote id="F10">The Form 4 filed for Mr. McNeil on January 4, 2016 incorrectly disclosed The Number of Derivative Securities Beneficially Owned Following Reported Transaction in Column 9 of Table II in units of securities as opposed to U.S. Dollars. This amended Form 4 corrects that error and appropriately disclosed the total in U.S. Dollars, which total includes 6.00% Convertible Debentures due December 31, 2019 previously acquired by Mr. McNeil.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s John S. Miele, attorney-in-fact</signatureName>
        <signatureDate>2016-01-05</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SUBMISSION>
