Exhibit 99.1

S&T Bancorp, Inc. Announces First Quarter Earnings

Indiana, Pennsylvania – S&T Bancorp, Inc. (NASDAQ: STBA), a full-service financial institution with office locations in 10 Pennsylvania counties, has announced its first quarter 2012 earnings.

Todd D. Brice, president and chief executive officer of S&T Bancorp, Inc., offered the following highlights:

“In the first quarter of 2012, we made several definitive moves to position ourselves for growth in the marketplace. We are very excited about the seamless closing and conversion of Mainline Bank and the announcement of our pending merger with Gateway Bank,” said Brice. “We have also applied additional resources to lines of business that show potential for growth, such as insurance and wealth management. Our first quarter results demonstrate that those investments are paying off.”

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S&T Bancorp, Inc. Announces First Quarter Earnings (continued)

Merger Activity

The first quarter of 2012 included one-time merger related expenses of $3.9 million, or $0.11 per share. This is in addition to one-time merger related expenses incurred in 2011 of $0.3 million. Both the holding company merger and the bank merger were completed on March 9, 2012, and the systems conversion occurred over the following weekend. Eight Mainline locations opened on March 12, 2012 as S&T Bank branches, adding approximately $130 million of loans and $205 million of deposits.

On March 30, 2012, S&T Bancorp, Inc. announced plans to merge with Gateway Bank of Pennsylvania. The in-market cash and stock deal is expected to close in the third quarter of 2012, and will expand S&T’s existing footprint in the northern and southern suburbs of Pittsburgh. Gateway has assets of $120 million and has two branches, including the headquarters branch in McMurray as well as a branch in Cranberry Township.

Net Interest Income

Net interest income and net interest margin on a fully taxable equivalent (FTE) basis decreased from the prior quarter and the last year quarter due to a continued shift in asset mix from loans to lower yielding securities and cash combined with lower loan rates on new business. Net interest income for the first quarter of 2012 declined to $34.5 million from $35.2 million in the fourth quarter of 2011 as the net interest margin decreased by 10 bps to 3.69%. Average loans increased by $9.4 million from the fourth quarter of 2011, however, this includes the first quarter impact of the Mainline merger of approximately $33.4 million in average loans. The decrease in average loans not including the Mainline merger is due to a continued high level of loan payoffs despite increases in new activity and an improving loan pipeline.

Asset Quality

Asset quality metrics exhibited some weakness this quarter, especially in the construction portfolio. Net charge-offs for the first quarter of 2012 were up significantly to $10.3 million, compared to $5.0 million in the fourth quarter of 2011 and only $0.4 million in

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S&T Bancorp, Inc. Announces First Quarter Earnings (continued)

the first quarter of 2011. Over half of the net charge-offs this quarter were related to construction projects that have experienced slowdowns and lower values on updated appraisals. Nonperforming assets increased to $67.9 million or 2.12% of total loans plus OREO compared to $60.1 million or 1.92% in the fourth quarter of 2011. The longer term asset quality trends continue to be positive, as nonperforming assets are down from $88.5 million or 2.67% in the first quarter of 2011. The provision for loan losses increased in the first quarter of 2012 to $9.3 million from $2.3 million in the fourth quarter of 2011, but is down from $10.6 million in the first quarter of 2011. The allowance for loan losses at March 31, 2011 was $47.8 million or 1.49% of total loans including acquired Mainline loans. This is little changed from the $48.8 million or 1.56% at December 31, 2011, but is down from $61.7 million or 1.87% at March 31, 2011.

Noninterest Income and Expense

Noninterest income showed improvement over both the fourth quarter of 2011 and the first quarter of 2011 due in large part to increases in Wealth Management and Insurance, as new producers have been added in both of these lines of business. Mortgage banking activity, which is included in Other Noninterest Income, improved in the second half of 2011 as rates declined, and has remained strong through the first quarter of 2012.

Noninterest expense was elevated in the first quarter of 2012 primarily due to the aforementioned one-time merger related expenses of $3.9 million. Salaries and Benefits includes one-time merger related expenses of $1.7 million and were also higher compared to the fourth quarter of 2011 due to annual merit increases of $0.4 million, higher pension expense of $0.5 million and higher payroll expense of $0.7 million, which is typically higher in the first quarter. Data Processing in the first quarter of 2012 included the buyout of Mainline’s remaining contract and Other Noninterest Expense included marketing and various professional expenses related to the merger. Also included in Other is an increase in the unfunded commitment reserve of $0.3 million compared to a decline of $0.5 million in the fourth quarter of 2011.

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S&T Bancorp, Inc. Announces First Quarter Earnings (continued)

Financial Condition

The size of the balance sheet increased over the fourth quarter of 2011 to $4.3 billion in total assets from $4.1 billion due to the merger with Mainline, which added $130 million of loans and $205 million of deposits.

S&T’s capital ratios were little changed due to the merger and flat retained earnings from modest net income. All capital ratios remain significantly above the well-capitalized thresholds of federal bank regulatory agencies with a tier 1 leverage capital ratio of 9.20%, tier 1 risk-based capital ratio of 11.62% and total risk-based capital ratio of 15.14%. With the redemption of the $108.7 million of preferred stock from the U.S. Department of Treasury’s Capital Purchase Program in the fourth quarter of 2011, S&T paid no preferred stock dividends in the first quarter of 2012, an after tax improvement of almost $1.6 million this quarter.

About S&T Bancorp, Inc.

Headquartered in Indiana, PA, S&T Bancorp, Inc. operates offices within Allegheny, Armstrong, Blair, Butler, Cambria, Clarion, Clearfield, Indiana, Jefferson and Westmoreland counties. With assets of $4.3 billion, S&T Bancorp, Inc. stock trades on the NASDAQ Global Select Market System under the symbol STBA. For more information, visit www.stbancorp.com.

This information may contain forward-looking statements regarding future financial performance which are not historical facts and which involve risks and uncertainties. Actual results and performance could differ materially from those anticipated by these forward-looking statements. Factors that could cause such a difference include, but are not limited to, general economic conditions, change in interest rates, deposit flows, loan demand, asset quality, including real estate and other collateral values and competition. In addition to the results of operations presented in accordance with Generally Accepted Accounting Principles (GAAP), S&T management uses and this press release contains or references, certain non-GAAP financial measures, such as net interest income on a fully taxable equivalent basis. S&T believes these non-GAAP financial measures provide information useful to investors in understanding our underlying operational performance and our business and performance trends as they facilitate comparisons with the performance of others in the financial services industry. Although S&T believes that these non-GAAP financial measures enhance investors’ understanding of S&T’s business and performance, these non-GAAP financial measures

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S&T Bancorp, Inc. Announces First Quarter Earnings (continued)

should not be considered an alternative to GAAP. A reconciliation of these non-GAAP financial measures is presented in the attached financial data spreadsheet. This information should be read in conjunction with the audited financial statements and analysis as presented in the Annual Report on Form 10-K for S&T Bancorp, Inc. and subsidiaries.

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S&T Bancorp, Inc.

Consolidated Selected Financial Data

Unaudited

(in thousands, except per share data)

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S&T Bancorp, Inc.

Consolidated Selected Financial Data

Unaudited

(in thousands)

S&T Earnings Release - 8

S&T Bancorp, Inc.

Consolidated Selected Financial Data

Unaudited

(in thousands)

S&T Earnings Release - 9

S&T Bancorp, Inc.

Consolidated Selected Financial Data

Unaudited

(in thousands)