ATI Inc.

Item 1.01 Entry into a Material Definitive Agreement On June 8, 2026, ATI Inc. (the “Company”) completed its offering and sale of $450 million aggregate principal amount of the Company’s unsecured 5.875% Senior Notes due 2033 (the “Notes”). The offering and sale of the Notes (the “Offering”) were made pursuant to the Company’s shelf registration statement filed with the Securities and Exchange Commission. The Notes were issued pursuant to the Indenture, dated as of September 14, 2021 (the “Indenture”), between the Company and Computershare Trust Company, N.A., as successor trustee to Wells Fargo Bank, National Association, as trustee (the “Trustee”), as supplemented by the Third Supplemental Indenture, dated as of June 8, 2026 (the “Supplemental Indenture”), between the Company and the Trustee. The Notes will accrue interest at the rate of 5.875% per annum and will mature on June 15, 2033. The Notes will be payable in cash semi-annually in arrears on each June 15 and December 15, commencing December 15, 2026. Prior to June 15, 2029, the Company may redeem the Notes at its option, at any time in whole or from time to time in part, at a redemption price equal to 100% of the principal amount of the Notes redeemed plus an applicable premium determined as set forth in the Supplemental Indenture as of, and accrued and unpaid interest to but not including, the redemption date (subject to the right of holders of record on the relevant record date to receive interest due on the relevant interest payment date). On and after June 15, 2029, the Company may redeem the Notes, at its option, at any time in whole or from time to time in part, at redemption prices specified in the Supplemental Indenture, plus accrued and unpaid interest to but not including the redemption date on the principal amount of the Notes to be redeemed. In addition, at any time prior to June 15, 2029, the Company may, at its option and at any time, redeem up to 35% of the aggregate principal amount of the Notes at a redemption price equal to 105.875% of the aggregate principal amount thereof, plus accrued and unpaid interest to but not including the redemption date on the principal amount of the Notes to be redeemed, with the net proceeds of one or more of certain equity offerings; provided that at least 65% of the aggregate principal amount of the Notes remain outstanding immediately after the occurrence of each such redemption. If an event of default with respect to the Notes occurs, the principal amount of the Notes, plus premium, if any, and accrued and unpaid interest may be declared immediately due and payable, subject to certain conditions. These amounts automatically become due and payable in the case of certain types of bankruptcy, insolvency or reorganization events of default involving the Company. The foregoing is a summary of the material terms and conditions of the Indenture, as supplemented by the Supplemental Indenture, and is not a complete discussion. Accordingly, the foregoing is qualified in its entirety by reference to the full text of the Indenture and the Supplemental Indenture, which are filed as Exhibit 4.1 and Exhibit 4.2, respectively, to this Current Report on Form 8-K, each of which is incorporated herein by reference. A form of Note is included in Exhibit 4.2.

Item 2.03 Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of a Registrant The disclosure set forth above under Item 1.01 is hereby incorporated by reference into this Item 2.03.

Item 8.01 Other Events On June 3, 2026, the Company executed and delivered an underwriting agreement (the “Underwriting Agreement”), by and between the Company and Goldman Sachs & Co. LLC, as representative of the underwriters named therein (the “Underwriters”), relating to the Offering. Under the terms of the Underwriting Agreement, the Company has agreed to indemnify the Underwriters against certain liabilities, including liabilities under the Securities Act of 1933, as amended, or to contribute payments that the Underwriters may be required to make because of any of those liabilities. A copy of the Underwriting Agreement is filed herewith as Exhibit 1.1 and is incorporated herein by reference. The opinion of the Company’s counsel as to the validity of the Notes issued and sold in the Offering is filed herewith as Exhibit 5.1.