S&T BANCORP INC

RECONCILIATIONS OF GAAP TONON-GAAP

In addition to traditional measures presented in accordance with GAAP, S&T’s management uses, and this Proxy Statement contains or references, certainnon-GAAP financial measures identified below. We believe thesenon-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 other companies in the financial services industry. Although we believe that thesenon-GAAP financial measures enhance investors’ understanding of our business and performance, thesenon-GAAP financial measures should not be considered an alternative to GAAP or considered to be more important than financial results determined in accordance with GAAP, nor are they necessarily comparable withnon-GAAP measures which may be presented by other companies.

Return on average tangible shareholders’ equity is a key profitability metric used by management to measure financial performance. The following table provides a reconciliation of return on average tangible shareholders’ equity(non-GAAP) by reconciling net income (GAAP) per the Consolidated Statements of Net Income to net income before amortization and intangibles and average shareholder’s equity to average tangible shareholders’ equity for the periods presented:

Pre-provision net revenue to average assets(non-GAAP) is a key profitability metric used by management to measure financial performance. We believe this to be a preferred industry measurement to help evaluate our ability to fund credit losses or build capital. The following table provides a reconciliation of income before taxes (GAAP) per the Consolidated Statements of Net Income to PPNR(non-GAAP) and then PPNR(non-GAAP) to average assets for the periods presented:

ANNUAL MEETING OF SHAREHOLDERS OF S&T BANCORP, INC. May 12, 2026 IMPORTANT NOTICE REGARDING THE AVAILABILITY OF PROXY MATERIALS FOR THE 2026 ANNUAL MEETING OF SHAREHOLDERS TO BE HELD ON MAY 12, 2026 S&T’s Proxy Statement for the 2026 Annual Meeting of Shareholders and S&T’s Annual Report on Form 10-K for the fiscal year ended December 31, 2025 are available at http://proxyvote.com Please sign, date and mail your proxy card in the envelope provided as soon as possible. Please detach along perforated line and mail in the envelope provided. V84001-P46471 REVOCABLE PROXY S&T BANCORP, INC. ANNUAL MEETING OF SHAREHOLDERS HELD ON MAY 12, 2026 THIS PROXY IS SOLICITED BY THE BOARD OF DIRECTORS The undersigned hereby appoints Michael Golden and Jeffrey J. Sottile or either of them, or any successors, with full powers of substitution, to act as attorneys and proxies for the undersigned to vote all shares of the common stock of S&T Bancorp, Inc. (“S&T”), par value $2.50 per share (“S&T Common Stock”), which the undersigned is entitled to vote at the Annual Meeting of Shareholders (the “Meeting”), to be held via Virtual Shareholder Meeting (www.virtualshareholdermeeting.com/STBA2026) on May 12, 2026 at 10:00 a.m., Eastern Time, and at any and all adjournments thereof, as indicated on the reverse hereof. The undersigned acknowledges receipt from S&T prior to execution of this proxy of the Notice of Meeting and the Proxy Statement. The undersigned hereby revokes any and all proxies heretofore given with respect to the undersigned’s shares of S&T Common Stock. This proxy, when properly executed, will be voted in the manner directed herein. If no such direction is made, this proxy will be voted in accordance with the Board of Directors’ recommendations. PLEASE SIGN, DATE AND RETURN PROMPTLY IN THE ENCLOSED ENVELOPE. (Continued and to be signed on the reverse side)

ANNUAL MEETING OF SHAREHOLDERS OF S&T BANCORP, INC. May 12, 2026 IMPORTANT NOTICE REGARDING THE AVAILABILITY OF PROXY MATERIALS FOR THE 2026 ANNUAL MEETING OF SHAREHOLDERS TO BE HELD ON MAY 12, 2026 S&T’s Proxy Statement for the 2026 Annual Meeting of Shareholders and S&T’s Annual Report on Form 10-K for the fiscal year ended December 31, 2025 are available at http://proxyvote.com Please sign, date and mail your proxy card in the envelope provided as soon as possible. Please detach along perforated line and mail in the envelope provided. V84001-P46471 REVOCABLE PROXY S&T BANCORP, INC. ANNUAL MEETING OF SHAREHOLDERS HELD ON MAY 12, 2026 THIS PROXY IS SOLICITED BY THE BOARD OF DIRECTORS The undersigned hereby appoints Michael Golden and Jeffrey J. Sottile or either of them, or any successors, with full powers of substitution, to act as attorneys and proxies for the undersigned to vote all shares of the common stock of S&T Bancorp, Inc. (“S&T”), par value $2.50 per share (“S&T Common Stock”), which the undersigned is entitled to vote at the Annual Meeting of Shareholders (the “Meeting”), to be held via Virtual Shareholder Meeting (www.virtualshareholdermeeting.com/STBA2026) on May 12, 2026 at 10:00 a.m., Eastern Time, and at any and all adjournments thereof, as indicated on the reverse hereof. The undersigned acknowledges receipt from S&T prior to execution of this proxy of the Notice of Meeting and the Proxy Statement. The undersigned hereby revokes any and all proxies heretofore given with respect to the undersigned’s shares of S&T Common Stock. This proxy, when properly executed, will be voted in the manner directed herein. If no such direction is made, this proxy will be voted in accordance with the Board of Directors’ recommendations. PLEASE SIGN, DATE AND RETURN PROMPTLY IN THE ENCLOSED ENVELOPE. (Continued and to be signed on the reverse side)