On March 20, 2019, Allegheny Technologies Incorporated (the Company) filed
with the Securities and Exchange Commission a definitive proxy statement (the Proxy Statement) for the Companys 2019 Annual Meeting of Shareholders, to be held on May 9, 2019 (the Annual Meeting). This supplement
is being filed to correct certain disclosures in the Executive CompensationCEO Pay Ratio section in the Proxy Statement to reflect the correction of an inadvertent calculation error.
The section entitled Executive CompensationCEO Pay Ratio on page 62 of the Proxy Statement is amended in its entirety as set
forth below. Except as specifically revised by the information contained herein, this supplement does not modify, amend or otherwise affect any of the other information set forth in the Proxy Statement. This supplement should be read with the Proxy
Statement, and, from and after the date of this supplement, any references to the Proxy Statement shall be deemed to include the Proxy Statement as supplemented hereby.
If you have already voted, you do not need to vote again unless you would like to change or revoke your prior vote on any proposal. If you
would like to change or revoke your prior vote on any proposal, please refer to the Proxy Statement for instructions on how to do so. If you have not already voted or if you wish to change your vote, please vote by telephone or internet to ensure
that your voting instructions are timely received before the Annual Meeting. Capitalized terms used in this Supplement and not otherwise defined have the meaning given to such terms in the Proxy Statement.
Pursuant to the Securities and Exchange Commissions Notice and Access rules, the Company is furnishing proxy materials to
its stockholders primarily via the Internet. Beginning on or about March 20, 2019, we mailed to our stockholders a Notice of Internet Availability of Proxy Materials (the Notice), containing instructions on how to access our proxy
materials on the Internet, including our Proxy Statement and our 2018 Annual Report. The Notice also instructs you on how you can vote using the Internet. Other stockholders, in accordance with their prior requests, have been mailed paper copies of
our proxy materials and a proxy card or voting form.
Important Notice Regarding the Availability of Proxy Materials
for the Annual Meeting to Be Held on May 9, 2019:
As permitted under applicable SEC rules, we mailed to 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 2018 Annual Report, as well as this Supplement, are available for review at:
envisionreports.com/ATI
.