Sec Form 4 Filing - Harris Timothy J @ ATI INC - 2026-01-05

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Harris Timothy J 2. Issuer Name and Ticker or Trading Symbol ATI INC [ ATI] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director _____ 10% Owner X __ Officer (give title below) _____ Other (specify below) Senior VP and CDIO (Last) (First) (Middle) C/O ATI INC., 2021 MCKINNEY AVE., SUITE 1100 3. Date of Earliest Transaction (MM/DD/YY) 01/05/2026 (Street) DALLAS, TX75201 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock, par value $0.10 per share 01/05/2026 A 2,677 ( 1 ) A $ 0 ( 2 ) 100,987 D Common Stock, par value $0.10 per share 01/05/2026 M 70,442 A $ 0 ( 3 ) 171,429 D Common Stock, par value $0.10 per share 01/05/2026 F 30,918 ( 4 ) D $ 121.08 ( 5 ) 140,511 D Common Stock, par value $0.10 per share 01/05/2026 A 46,318 ( 6 ) A $ 0 ( 2 ) 186,829 D Common Stock, par value $0.10 per share 01/05/2026 F 20,283 ( 7 ) D $ 121.08 ( 5 ) 166,546 D Common Stock, par value $0.10 per share 01/05/2026 F 3,359 ( 8 ) D $ 121.08 ( 5 ) 163,187 D

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Performance Stock Unit $ 0 ( 3 ) 01/05/2026 M 25,641 12/31/2026( 3 ) 12/31/2026( 3 ) Common Stock, par value $0.10 per share 70,442 $ 0 ( 3 ) 25,641 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Harris Timothy J C/O ATI INC. 2021 MCKINNEY AVE., SUITE 1100 DALLAS, TX75201 Senior VP and CDIO Signatures /s/ Amanda J. Skov, Attorney-in-Fact for Timothy J. Harris 01/07/2026 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Award of restricted stock units, which are settled in shares of stock upon vesting. The award vests in three equal annual installments on each of the first three anniversaries of the grant date. ( 2 )Awarded under the Issuer's 2022 Incentive Plan. ( 3 )Settlement of one-half of certain Performance Stock Units awarded in 2022 ("2022 Breakout Performance Units"), each of which represented a contingent right to receive shares of the Issuer's Common Stock if the Issuer's Common Stock achieved specified target market prices (based on a 20-trading day average) on the New York Stock Exchange ("NYSE") for at least 20 consecutive trading days (the "20-day Average Market Price") prior to December 31, 2025, up to a maximum of three shares per Unit. The shares underlying one-half of the 2022 Breakout Performance Units became payable as of January 5, 2026 following certification of the Issuer's achievement of the relevant performance criteria by the Compensation and Leadership Development Committee of the Company's Board of Directors (the "CLDC"). The remaining half of the 2022 Breakout Performance Units are scheduled by their terms to become payable in early 2027. ( 4 )Shares withheld for the payment of taxes in connection with the settlement of 2022 Breakout Performance Units. ( 5 )Represents the average of the high and low trading prices for one share of the Issuer's Common Stock on the NYSE on January 5, 2026. ( 6 )Settlement of performance-vested restricted stock units that were granted in 2023, the vesting of which was contingent on the Issuer's total shareholder return relative to a specified peer group during the period from January 1, 2023 through December 31, 2025 (the "2023-2025 PSUs"). The shares underlying the 2023-2025 PSUs became payable as of January 5, 2026 following certification of the Issuer's achievement of the relevant performance criteria by the CLDC. ( 7 )Shares withheld for the payment of taxes in connection with the settlement of the 2023-2025 PSUs. ( 8 )Shares withheld for payment of taxes on restricted stock units awarded in 2023, 2024 and 2025. One third of each such award vested by the terms thereof on January 5, 2026.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

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