UNITED STATES SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the Securities

Exchange Act of 1934

Filed by the Registrant þ

Filed by a Party other than the Registrant ¨

Check the appropriate box:

¨ Preliminary Proxy Statement

¨ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

þ Definitive Additional Materials

¨ Soliciting Material Pursuant to §240.14a-12

AEROJET ROCKETDYNE HOLDINGS, INC.

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check the appropriate box):

þ No fee required.

¨ Fee computed on table below per Exchange Act Rules 14a-6(i)(4) and 0-11.

1) Title of each class of securities to which transaction applies:

2) Aggregate number of securities to which transaction applies:

3) Per unit price or other underlying value of transaction computed pursuant to Exchange Act Rule 0-11 (set forth the amount on which the filing fee is calculated and state how it was determined):

4) Proposed maximum aggregate value of transaction:

5) Total fee paid:

¨ Fee paid previously with preliminary materials.

¨ Check box if any part of the fee is offset as provided by Exchange Act Rule 0-11(a)(2) and identify the filing for which the offsetting fee was paid previously. Identify the previous filing by registration statement number, or the Form or Schedule and the date of its filing.

1) Amount Previously Paid:

2) Form, Schedule or Registration Statement No.:

3) Filing Party:

4) Date Filed:

*** Exercise Your Right to Vote ***

Important Notice Regarding the Availability of Proxy Materials for the Stockholder Meeting to Be Held on April 27, 2016.

AEROJET ROCKETDYNE HOLDINGS, INC.

AEROJET ROCKETDYNE HOLDINGS, INC.

JOANNE KEETER

P.O. BOX 537012

SACRAMENTO, CA 95853-7012

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Meeting Information

Meeting Type: Annual Meeting

For holders as of: March 1, 2016

Date: April 27, 2016 Time: 9:00 AM PDT

Location: Meeting live via the Internet-please visit ajrd.onlineshareholdermeeting.com.

The company will be hosting the meeting live via the Internet this year. To attend the meeting via the Internet please visit ajrd.onlineshareholdermeeting.com and be sure to have the information that is printed in the box marked by the arrow (located on the following page).

You are receiving this communication because you hold shares in the company named above.

This is not a ballot. You cannot use this notice to vote these shares. This communication presents only an overview of the more complete proxy materials that are available to you on the Internet. You may view the proxy materials online at www.proxyvote.com or easily request a paper copy (see reverse side).

We encourage you to access and review all of the important information contained in the proxy materials before voting.

See the reverse side of this notice to obtain proxy materials and voting instructions.

Before You Vote

How to Access the Proxy Materials

How To Vote

Please Choose One of the Following Voting Methods

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Voting Items

The Board of Directors recommends you vote FOR the following:

1. Election of Directors

Nominees:

The Board of Directors recommends you vote FOR the following proposals:

2. To consider and approve an advisory resolution to approve executive compensation.

3. To ratify the appointment of PricewaterhouseCoopers LLP, an independent registered public accounting firm, as independent auditors of the Company for the fiscal year ending December 31, 2016.

NOTE: To consider and act on such other business as may properly be brought before the meeting or any adjournments or postponements thereof.

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