Community Bank System (CBU) 10-Q 2024 Q3 Quarterly report

Exhibit 32.1

CERTIFICATION PURSUANT TO

18 U.S.C. SECTION 1350,

AS ADOPTED PURSUANT TO

SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

In connection with the Quarterly Report of Community Financial System, Inc. (the “Company”) on Form 10-Q for the quarter ended September 30, 2024 as filed with the Securities and Exchange Commission on the date hereof (the “Report”), I, Dimitar A. Karaivanov, President and Chief Executive Officer of the Company, certify, pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, that to the best of my knowledge:

(1)the Report fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and

(2)the information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Company.

This certification accompanies this Form 10-Q and shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, or otherwise subject to the liability of that Section.

/s/ Dimitar A. Karaivanov

Dimitar A. Karaivanov

President and Chief Executive Officer

November 12, 2024