Financial Institutions (FISI) 10-K 2016 FY Annual report

Exhibit 23

Consent of Independent Registered Public Accounting Firm

The Board of Directors and Shareholders

Financial Institutions, Inc.:

We consent to incorporation by reference in the Registration StatementsNo. 333-40544,333-82202,333-87338,333-172651,333-172652 and333-204446 onForm S-8 of Financial Institutions, Inc. of our reports, dated March 7, 2017, relating to the consolidated statements of financial condition of Financial Institutions, Inc. and subsidiaries (the Company) as of December 31, 2016 and 2015, and the related consolidated statements of income, comprehensive income, changes in shareholders’ equity, and cash flows for each of the years in the three-year period ended December 31, 2016, and the effectiveness of internal control over financial reporting as of December 31, 2016, which reports appear in the December 31, 2016 annual report onForm 10-K of Financial Institutions, Inc.

/s/ KPMG LLP

Rochester, New York

March 7, 2017