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UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , DC 20549 FORM 10 - K ANNUAL REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 , TRANSITION REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 , for the Fiscal Year Ended December 31 , 2020 , or for the transition period from Delaware ( State or other jurisdiction of incorporation or organization ) BROOKLINE BANCORP , INC . ( Exact name of registrant as specified in its charter ) 131 Clarendon Street ( Address of principal executive offices ) N / A to Commission File Number : 0-23695 Title of Each Class Common Stock , par value of $ 0.01 per share Boston MA Securities registered pursuant to Section 12 ( b ) of the Act : Large accelerated filer Non - accelerated filer 04-3402944 ( 617 ) 425-4600 ( Registrant's telephone number , including area code ) Trading Symbol ( s ) BRKL ( I.R.S. Employer Identification No. ) 02116 ( Zip Code ) Name of Each Exchange on Which Registered Nasdaq Global Select Market Securities registered pursuant to Section 12 ( g ) of the Act : None Indicate by check mark if the registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act of 1934. Yes No > Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or 15 ( d ) of the Securities Act of 1934. Yes No X Indicate by check mark whether the registrant ( 1 ) has filed all reports required to be filed by Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 during the preceding 12 months ( or for such shorter period that the registrant was required to file such reports ) and ( 2 ) has been subject to such filing requirement for the past 90 days . Yes No Indicate by check mark whether the registrant ( 1 ) has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S - T ( §232.405 of this chapter ) during the preceding 12 months ( or for such shorter period that the registrant was required to submit such files ) . Yes No Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer , a smaller reporting company , or emerging growth company . See the definitions of " large accelerated filer " , " accelerated filer " , " smaller reporting company " , and " emerging growth company " in Rule 12b - 2 of the Exchange Act . Accelerated filer Smaller Reporting Company Emerging growth company If an emerging growth company , indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13 ( a ) of the Exchange Act . Indicate by check mark whether the registrant has filed a report on and attestation to its management's assessment of the effectiveness of its internal control over financial reporting under Section 404 ( b ) of the Sarbanes - Oxley Act ( 15 U.S.C. 7262 ( b ) ) by the registered public accounting firm that prepared or issued its audit report . Indicate by check mark if the registrant is a shell company ( as defined in Rule 12b - 2 of the Act ) . Yes No