Exhibit 23.1

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CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

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We hereby consent to the incorporation by reference in the Registration Statements on Form S-8 (Nos. 333-119590, 333-119887, 333-185556, 333-221190, 333-266241, 333-280857, and 333-280858) of Community Financial System, Inc. of our report dated February 28, 2025, relating to the financial statements and the effectiveness of internal control over financial reporting, which appears in this Form 10-K.

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/s/PricewaterhouseCoopers LLP

Buffalo, New York

February 28, 2025