ATI INC

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

(Rule

14a-101)

INFORMATION REQUIRED IN PROXY STATEMENT

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant To Section 14(a) of the

Securities Exchange Act of 1934

Filed by the Registrant ☒ Filed by a Party other than the Registrant ☐

Check the appropriate box:

ATI Inc. 2021 McKinney Avenue Dallas, TX 75201

DEAR STOCKHOLDERS

As Chair of your Board of Directors, I am pleased to invite you to attend ATI’s 2024 Annual Meeting of Stockholders.

In 2023, ATI achieved another year of accelerating growth and expanding leadership in our core aerospace and defense markets. We’ve deliberately transformed our business, investing to expand capacity and grow our capabilities. These decisive actions are enabling us to capitalize on valuable emerging opportunities. In 2023 alone, we captured more than $1.2 billion in new sales commitments from leading aerospace and defense customers, representing an average of $200 million per year in estimated revenue, 2024 to 2029. Our leadership in these rapidly growing markets sets the stage for sustained, multi-year growth.

We delivered more than $4 billion in revenue during 2023. With aerospace and defense sales increasing 32% year over year and accounting for 63% of total sales in the fourth quarter, we are on track to achieve our goal for 65% of sales attributable to these growing markets.

Also among our key 2023 achievements: we successfully executed a series of pensionde-risking strategies that reduced our U.S. defined benefit pension obligation by approximately 85%. The culmination of a multi-year effort, these actions enable us to fully meet our obligations to pension plan participants while mitigating volatility that impacted other stakeholders.

Simply put, we did what we said we would do. Our commitment to focused execution of our most impactful strategic priorities drove results for our shareholders. As you will see illustrated in this Proxy Statement, we are outpacing markets and peers in delivering stand-out shareholder return over time. A $100 investment in ATI at the outset of 2022 is worth nearly $300 as of the record date for the Annual Meeting.

I am delighted that Kimberly A. Fields will become ATI’s Chief Executive Officer in July. She’ll have the benefit of sustainable growth rooted in our strategic actions and will lead a business poised for a future of energizing opportunities to deliver shareholder value.

The time is right. Kim is proven to perform. With her deep operational expertise and commercial acumen, she’s been a key collaborator in the successful transformation of our business. I’m confident her strategic vision will continue to drive outstanding success for our stakeholders. Kim has also recently joined ATI’s Board of Directors. I look forward to being a valued resource to Kim and the ATI Board in my continuing role as Executive Chairman.

I am also pleased to welcome Vaishali Bhatia, our Senior Vice President, General Counsel and Chief Compliance Officer, as the

newest member of our executive leadership team. As a proven leader with deep experience in technically differentiated markets, Vaishali brings valuable new insights to our team. And, our commitment to diversity and inclusion is demonstrable: women and members of under-represented ethnic groups comprise more than 50% of our senior leadership team.

We also remain committed to reducing our carbon footprint and exploring the many opportunities the “greening” of the global economy presents to our business. We’ve reduced our own greenhouse gas emissions by nearly half since 2018, and with the publication of our next Corporate Responsibility Report in April, will announce a new, more significant 2030 carbon reduction goal.

Together, we remain rooted in our core values: we do what we say we’re going to do, when we say we’re going to do it, and we do it the right way. These values permeate our corporate governance practices, cultivation of our human capital and culture, investor outreach efforts and compensation programs. You’ll see that reflected in this Proxy Statement.

In summary, we are meeting our commitments through strong execution and operational excellence, and we’re getting better every day. As a result, our customers count on us to deliver the mission-critical materials and components they need. We are honored to be rewarded with more of their business.

I encourage you to participate in our virtual 2024 Annual Meeting. Information about how to join, including how to pose questions to our Board and management, is contained in this Proxy Statement. As we look to the future, your Board remains committed to serving as effective stewards of your investment and advocates for your interests and concerns. My fellow Board members and I value

feedback from our investors and look forward to our ongoing dialogue. Thank you for your support of ATI. Sincerely, Robert S. Wetherbee Board Chair and Chief Executive Officer March 28, 2024

Notice of Annual Meeting

of Stockholders

Annual Meeting Information

DATE & TIME: Thursday, May 16, 2024 11:30 a.m. Central Time

VIRTUAL MEETING SITE: www.meetnow.global/MJ4M9LH

Agenda

Your vote is important Please vote as soon as possible. You can help the Company reduce expenses by voting your shares by telephone or Internet; your proxy card or voting instruction card contains the instructions. Or complete, sign and date your proxy card or voting instruction card and return it as soon as possible in the enclosed postage-paid envelope. How to vote

Via the internet Visit thewebsite listed on ourproxy card

By mail Sign, date and returnyour proxy card in theenclosed envelope

By telephone Call thetelephone number on yourproxy card

During the Virtual Meeting Attend the Annual Meetingwhich is being presentedvirtually via webcast and voteonline during the meeting

Admission to the Meeting

You are entitled to participate, vote and submit questions at ATI Inc.’s virtual 2024 Annual Meeting if you were a stockholder of record as of the close of business on March 18, 2024, the record date, or hold a legal proxy for the meeting provided by your bank, broker or nominee.

Stockholders of Record. You will be able to participate in the 2024 Annual Meeting, vote electronically and submit questions during the live webcast of the meeting, without advance registration. To access the live webcast of the meeting you will need the 15-digit control number on the proxy card or the “Notice Regarding the Availability of Proxy Materials” (the “Notice”) you previously received.

Beneficial Owners. If you are a beneficial owner and hold your shares through an intermediary, such as a bank, broker, or nominee, you must register in advance to participate in the 2024 Annual Meeting, vote electronically and submit questions during the live webcast of the meeting. To register in advance, you must obtain a legal proxy from the bank, broker or other nominee that holds your shares giving you the right to vote the shares. You must forward a copy of the legal proxy, along with your email address, to Computershare Trust Company, N.A. (“Computershare”) by email atlegalproxy@computershare.com no later than 4:00 p.m. Central Time, on Monday, May 13, 2024. You will receive a confirmation of your registration and instructions on how to attend the meeting by email after Computershare receives your registration materials.

Beneficial owners that are unable to register in advance may still attend the 2024 Annual Meeting by visitingwww.meetnow.global/MJ4M9LHas a “guest” but will not have the option to vote shares electronically or submit questions during the live webcast of the meeting. However, you may submit questions in advance of the meeting by emailing your question, along with proof of ownership, to investors@ATlmaterials.com.

Our Proxy Statement and 2023 Annual Report are available for review by stockholders of record atenvisionreports.com/ATI and by beneficial owners atedocumentreview.com/ATI. For further information about ATI, please visit our website atatimaterials.com. This Proxy Statement is first being provided to our stockholders on or about March 28, 2024.

On behalf of the Board of Directors:

March 28, 2024

Important notice regarding the availability of proxy materials for the ATI Annual Meeting of Stockholders to be held on Thursday, May 16, 2024.

As permitted under applicable Securities and Exchange Commission (“SEC”) rules, we are mailing our stockholders a Notice of Internet Availability of Proxy Materials containing instructions on how to access our proxy materials and submit proxy votes online. Our Proxy Statement, proxy card and 2023 Annual Report are available for review at: envisionreports.com/ATI.