UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): May 15, 2017

(Exact name of Registrant as specified in its charter)

Registrant’s telephone number, including area code: (585) 786-1100

Not Applicable

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (17 CFR §230.405) or Rule 12b-2 of the Securities Exchange Act of 1934 (17 CFR §240.12b-2).

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.  ☐

On May 15, 2017, Financial Institutions, Inc. (the “Company”) issued a press release announcing the commencement of an underwritten public offering of $40.0 million of its common stock (the “Offering”). Sandler O’Neill + Partners, L.P. is serving as book-running manager for the Offering, and Hovde Group, LLC and Piper Jaffray & Co. are co-managers.

A copy of the Company’s press release is attached hereto as Exhibit 99.1. A copy of the investor presentation for the Offering, which is also being filed with the SEC as an issuer free writing prospectus, is attached hereto as Exhibit 99.2.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.