Exhibit 99.1

Financial Institutions, Inc.

FINANCIAL INSTITUTIONS, INC. ANNOUNCES THIRD QUARTER 2017 RESULTS

— Surpassed $4 Billion in Total Assets

WARSAW, N.Y., October 24, 2017 – Financial Institutions, Inc. (NASDAQ: FISI), today reported financial and operational results for the third quarter ended September 30, 2017. Financial Institutions, Inc. (the “Company”) is the parent company of Five Star Bank (the “Bank”), Scott Danahy Naylon, LLC (“Scott Danahy Naylon” or “SDN”) and Courier Capital, LLC (“Courier Capital”).

Net income for the quarter was $8.3 million, compared to $6.2 million for the second quarter of 2017 and $8.5 million for the third quarter of 2016. After preferred dividends, net income available to common shareholders was $7.9 million, or $0.52 per diluted share (“EPS”), compared to $5.9 million, or $0.40 per diluted share, for the second quarter of 2017 and $8.1 million, or $0.56 per diluted share, for the third quarter of 2016.

President and Chief Executive Officer Martin K. Birmingham stated, “It was a good quarter for our Company and we made significant progress on key initiatives. We sold nearly 500,000 shares of common stock under our ongoing at-the-market equity offering (“ATM Offering”), generating approximately $13.0 million of net proceeds. The capital raised through the offering positions us to continue to take advantage of growth opportunities in our geographic footprint. In late August, we acquired the assets of a Buffalo-area wealth management firm, furthering our strategy to increase fee-based noninterest income and diversify revenues. This bolt-on acquisition increases Courier Capital’s total assets under management to approximately $1.6 billion and expands its client base in Western New York.

“We are also pleased to announce that we have achieved the important milestone of surpassing $4 billion in total assets. This accomplishment was achieved through the collective performance of our team and the successful execution of our strategic plan.”

Chief Financial Officer Kevin B. Klotzbach added, “We continued to generate loan growth in the third quarter of 2017 — total loans were 3.9% higher than the prior quarter, including 4.7% growth in commercial mortgage loans, 5.3% growth in commercial business loans and 3.7% growth in consumer indirect loans. In a challenging interest rate environment, we maintained a stable net interest margin. Our margin of 3.17% in the third quarter was one basis point lower than the prior quarter, primarily as a result of the higher cost of short-term borrowings.

“We are pleased with the progress made to date on our ATM offering. The issuance of common stock has positively impacted our tangible common equity to tangible assets ratio; however, it also negatively impacted EPS by approximately two cents in the quarter.”

Third Quarter 2017 Highlights:

Net interest income of $28.4 million increased $2.4 million, or 9.2%, as compared to the third quarter of 2016

Noninterest income of $8.6 million was $35 thousand, or 0.4%, higher than the third quarter of 2016

Excluding the net gain on investment securities from both periods, noninterest income was $8.4 million, 3.4% higher than the third quarter of 2016

Total interest-earning assets, assets, loans and deposits all reached record-high levels at quarter-end:

Total interest-earning assets increased $115.3 million during the quarter, to $3.71 billion

Total assets increased $130.1 million during the quarter, to $4.02 billion

Total loans increased $99.4 million during the quarter, to $2.62 billion

Total deposits increased $149.0 million during the quarter, to $3.28 billion

The Company declared a quarterly cash dividend of $0.21 per common share, which represented a 2.89% annualized dividend yield as of September 30, 2017, and a return of 40% of third quarter net income to common shareholders

The Company made significant progress on its priority to grow Five Star Bank’s residential mortgage lending business

The Company continued its ATM Offering and sold 498,038 common shares, generating $13.5 million of gross proceeds ($13.0 million of net proceeds)

Courier Capital acquired the assets of Robshaw & Julian Associates, a Western New York investment advisory firm

“At-The-Market” Offering of Common Stock

On May 30, 2017, the Company announced an ATM Offering program under which it may sell up to $40 million of its common stock. The Company expects to use the net proceeds of this offering to support organic growth and other general corporate purposes, including contributing capital to its banking subsidiary, Five Star Bank. To date, the Company has sold 1,069,635 shares of its common stock under this program at a weighted average price of $29.01, representing gross proceeds of $31.0 million. Net proceeds received were $29.7 million.

Acquisition of Robshaw & Julian Associates

On August 31, 2017, Courier Capital acquired the assets of Robshaw & Julian Associates, Inc., an investment advisor based in Williamsville, New York. The firm’s assets under management (“AUM”) totaled approximately $175 million, increasing Courier Capital’s AUM after closing to approximately $1.6 billion. The prior owners of the firm have been named officers of Courier Capital, where they are expected to continue to manage their portfolios.

Net Interest Income and Net Interest Margin

Net interest income was $28.4 million for the third quarter of 2017, $1.0 million higher than the second quarter of 2017 and $2.4 million higher than the third quarter of 2016.

Average interest-earning assets for the quarter were $3.67 billion, $111.7 million higher than the second quarter of 2017 and $351.4 million higher than the third quarter of 2016. The primary driver of the increase was organic loan growth.

Third quarter 2017 net interest margin was 3.17%, one basis point lower than the second quarter of 2017 and six basis points lower than the third quarter of 2016. Net interest margin has been negatively impacted by a flattening of the yield curve and higher short-term borrowing costs.

Noninterest Income

Noninterest income was $8.6 million for the third quarter of 2017 as compared to $9.3 million in the second quarter of 2017 and $8.5 million in the third quarter of 2016.

Excluding the net gain on investment securities from all periods, noninterest income was $8.4 million in the third quarter of 2017, $733 thousand lower than $9.1 million in the second quarter of 2017, and $277 thousand higher than $8.1 million in the third quarter of 2016.

Lower noninterest income in the third quarter of 2017 as compared to the second quarter of 2017 was primarily the result of a second quarter non-recurring $1.2 million non-cash fair value adjustment of contingent consideration liability relating to SDN. Partially offsetting this decrease was a $355 thousand increase in insurance income due to the timing of customer renewals.

Higher noninterest income in the third quarter of 2017 as compared to the third quarter of 2016 was primarily the result of a $171 thousand increase in investment advisory income and the recognition of a $127 thousand fair value adjustment to our derivative financial instruments.

Noninterest Expense

Noninterest expense was $22.5 million for the third quarter of 2017 as compared to $23.9 million in the second quarter of 2017 and $20.6 million in the third quarter of 2016.

Noninterest expense decreased from the second quarter of 2017 primarily as a result of a second quarter non-recurring $1.6 million non-cash goodwill impairment charge relating to SDN.

The increase in noninterest expense as compared to the third quarter of 2016 was primarily the result of higher salaries and employee benefits, occupancy and equipment expenses, and computer and data processing expenses related to our organic growth initiatives.

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Income Taxes

Income tax expense was $3.5 million for the third quarter of 2017 as compared to $2.7 million in the second quarter of 2017 and $3.5 million in the third quarter of 2016. The effective tax rate was 29.5% for the third quarter of 2017, 30.5% in the second quarter of 2017, and 29.5% in the third quarter of 2016. The higher effective tax rate in the second quarter of 2017 was a result of the $1.6 million non-cash goodwill impairment charge, partially offset by the $1.2 million non-cash fair value adjustment of contingent consideration liability, both of which were non-taxable adjustments related to our 2014 acquisition of SDN.

Balance Sheet and Capital Management

Total assets were $4.02 billion at September 30, 2017, up $130.1 million from $3.89 billion at June 30, 2017, and up $311.3 million from $3.71 billion at December 31, 2016. The increases were primarily the result of loan growth funded by deposit growth.

Total loans were $2.62 billion at September 30, 2017, up $99.4 million, or 3.9%, from June 30, 2017, and up $276.1 million, or 11.8%, from December 31, 2016.

Commercial business loans totaled $419.4 million, up $21.1 million, or 5.3%, from June 30, 2017, and up $69.9 million, or 20.0%, from December 31, 2016.

Commercial mortgage loans totaled $758.0 million, up $33.9 million, or 4.7%, from June 30, 2017, and up $87.9 million, or 13.1%, from December 31, 2016.

Residential real estate loans totaled $446.0 million, up $14.0 million, or 3.2%, from June 30, 2017, and up $18.1 million, or 4.2%, from December 31, 2016.

Consumer indirect loans totaled $857.5 million, up $30.8 million, or 3.7%, from June 30, 2017, and up $105.1 million, or 14.0%, from December 31, 2016.

Total deposits were $3.28 billion at September 30, 2017, an increase of $149.0 million from June 30, 2017, and an increase of $286.3 million from December 31, 2016. The increase from June 30, 2017, was primarily due to public deposit seasonality. The increase from December 31, 2016, was primarily the result of successful business development efforts in both municipal and retail banking. Public deposit balances represented 28% of total deposits at September 30, 2017, compared to 27% at June 30, 2017, and 27% at December 31, 2016. Nonpublic deposits increased 2.7% from June 30, 2017, and 7.5% from December 31, 2016.

Short-term borrowings were $310.8 million at September 30, 2017, down $36.7 million from June 30, 2017, and down $20.7 million from December 31, 2016. Short-term borrowings are typically utilized to manage the seasonality of public deposits.

Shareholders’ equity was $366.0 million at September 30, 2017, compared to $347.6 million at June 30, 2017, and $320.1 million at December 31, 2016. Common book value per share was $22.31 at September 30, 2017, an increase of $0.47 or 2.2% from $21.84 at June 30, 2017, and an increase of $1.49 or 7.2% from $20.82 at December 31, 2016. The increases in shareholders’ equity and common book value per share are attributable to common stock issued through our ATM Offering plus net income less dividends paid, net of the change in unrealized gain (loss) on investment securities.

During the third quarter 2017, the Company declared a common stock dividend of $0.21 per common share. The third quarter 2017 dividend returned 40% of third quarter net income to common shareholders.

Regulatory capital ratios at September 30, 2017, were higher than the prior quarter and prior year due to increased capital as a result of the recent ATM Offering:

Leverage Ratio was 7.91%, compared to 7.70% and 7.36% at June 30, 2017, and December 31, 2016, respectively.

Common Equity Tier 1 Ratio was 10.09%, compared to 9.86% and 9.59% at June 30, 2017, and December 31, 2016, respectively.

Tier 1 Risk-Based Capital was 10.69%, compared to 10.48% and 10.26% at June 30, 2017, and December 31, 2016, respectively.

Total Risk-Based Capital was 13.24%, compared to 13.09% and 12.97% at June 30, 2017, and December 31, 2016, respectively.

Credit Quality

Non-performing loans were $12.6 million at September 30, and June 30, 2017, and $6.3 million at December 31, 2016. The increase from December 31, 2016, was primarily the result of the second quarter of 2017 internal downgrade of two commercial credit relationships with unpaid principal balances totaling $5.6 million.

The provision for loan losses for the quarter was $2.8 million, a decrease of $1.0 million from the second quarter of 2017 and an increase of $841 thousand from the third quarter of 2016. The higher provision in the second quarter of 2017 was primarily attributable to the downgrade of one commercial credit relationship. The downgrade necessitated a provision and increase in allowance for loan losses of approximately $925 thousand. The increase in provision from the third quarter of 2016 is primarily attributable to growth in the total loan portfolio.

The ratio of annualized net charge-offs to total average loans was 0.25% in the current quarter, compared to 0.29% in the prior quarter and 0.20% in the third quarter of 2016.

The ratio of non-performing loans to total loans was 0.48% at September 30, 2017, compared to 0.50% at June 30, 2017, and 0.27% at December 31, 2016.

The ratio of allowance for loans losses to total loans was 1.31% at September 30, 2017, 1.32% at June 30, 2017, and 1.32% at December 31, 2016.

The ratio of allowance for loan losses to non-performing loans was 273% at September 30, 2017, 263% at June 30, 2017, and 489% at December 31, 2016.

About Financial Institutions, Inc.

Financial Institutions, Inc. provides diversified financial services through its subsidiaries, Five Star Bank, Scott Danahy Naylon and Courier Capital. Five Star Bank provides a wide range of consumer and commercial banking and lending services to individuals, municipalities and businesses through a network of more than 50 offices throughout Western and Central New York State. Scott Danahy Naylon provides a broad range of insurance services to personal and business clients across 45 states. Courier Capital provides customized investment management, investment consulting and retirement plan services to individuals, businesses, institutions, foundations and retirement plans. Financial Institutions, Inc. and its subsidiaries employ approximately 650 individuals. The Company’s stock is listed on the NASDAQ Global Select Market under the symbol FISI. Additional information is available at www.fiiwarsaw.com.

Non-GAAP Financial Information

This news release contains disclosure regarding tangible common equity, tangible common equity to tangible assets, tangible common book value per share, average tangible common equity and return on average tangible common equity, which are determined by methods other than in accordance with U.S. generally accepted accounting principles (“GAAP”). The Company believes that these non-GAAP measures are useful to our investors as measures of the strength of the Company’s capital and ability to generate earnings on tangible common equity invested by our shareholders. These non-GAAP measures provide supplemental information that may help investors to analyze our capital position without regard to the effects of intangible assets. Non-GAAP financial measures have inherent limitations and are not uniformly applied by issuers. Therefore, these non-GAAP financial measures should not be considered in isolation, or as a substitute for comparable measures prepared in accordance with GAAP. The comparable GAAP financial measures and reconciliation to the comparable GAAP financial measures can be found in Appendix A to this document.

Safe Harbor Statement

This press release may contain forward-looking statements as defined by Section 21E of the Securities Exchange Act of 1934, as amended, that involve significant risks and uncertainties. Statements herein are based on certain assumptions and analyses by the Company and are factors it believes are appropriate in the circumstances. Actual results could differ materially from those contained in or implied by such statements for a variety of reasons including, but not limited to: the Company’s ability to implement its strategic plan, the Company’s ability to redeploy investment assets into loan assets, whether the Company experiences greater credit losses than expected, whether the Company experiences breaches of its, or third party, information systems, the attitudes and preferences of the Company’s customers, the Company’s ability to successfully integrate and profitably operate Scott Danahy Naylon and Courier Capital, the competitive environment, fluctuations in the fair value of securities in its investment portfolio, changes in the regulatory environment and the Company’s compliance with regulatory requirements, changes in interest rates, general economic and credit market conditions nationally and regionally. Consequently, all forward-looking statements made herein are qualified by these cautionary statements and the cautionary language in the Company’s Annual Report on Form 10-K, its Quarterly Reports on Form 10-Q and other documents filed with the SEC.  Except as required by law, the Company undertakes no obligation to revise these statements following the date of this press release.

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