REGENXBIO Inc

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

(Rule 14a-101)

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

(Amendment No. )

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☐ Preliminary Proxy Statement

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☒ Definitive Proxy Statement

☐ Definitive Additional Materials

☐ Soliciting Material Pursuant to §240.14a-12

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Rockville, MD 20850

NOTICE OF ANNUAL MEETING OF STOCKHOLDERS

Dear Stockholder:

You are cordially invited to attend the Annual Meeting of Stockholders (the “Annual Meeting”) of REGENXBIO Inc., a Delaware corporation (the “Company”). The Annual Meeting will be held on May 31, 2024, at 9:00 a.m. local time at the Company’s offices located at 9804 Medical Center Drive, Rockville, Maryland 20850 for the following purposes:

To elect George Migausky, Kenneth T. Mills and David C. Stump, M.D., to serve as Class III directors until the 2027 annual meeting of stockholders;

To ratify the selection of PricewaterhouseCoopers LLP by the Audit Committee of the Board of Directors as the independent registered public accounting firm of the Company for the year ending December 31, 2024;

To hold an advisory vote on the compensation paid to the Company’s named executive officers; and

To transact any other business properly brought before the Annual Meeting and any adjournments or postponements thereof.

Only stockholders of record at the close of business on April 1, 2024 are entitled to notice of, and to vote at, the Annual Meeting or any adjournment or postponement thereof. A complete list of such stockholders will be available for inspection at the Company’s offices in Rockville, Maryland at the Annual Meeting and during normal business hours for a period of 10 days prior to the Annual Meeting.

Your vote is important. Whether or not you plan to attend the Annual Meeting in person, please submit your proxy by telephone or over the internet, or by completing, signing, dating and returning your proxy card or voting instruction form so that your shares will be represented at the Annual Meeting. Instructions for voting are described in the Company’s proxy statement for the Annual Meeting, Notice of Internet Availability of Proxy Materials and proxy card.

Important Notice Regarding the Availability of Proxy Materials for the Annual Meeting of Stockholders to be held on May 31, 2024:

The Company’s Notice of Annual Meeting, Proxy Statement and Annual Report on Form 10‑K for the fiscal year ended December 31, 2023 are available atwww.proxyvote.com.

By Order of the Board of Directors,

Kenneth T. MillsPresident and Chief Executive Officer

Rockville, MarylandApril 4, 2024

This Proxy Statement is first being mailed to the stockholders of the Company on or about April 4, 2024.

TABLE OF CONTENTS

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Rockville, MD 20850

PROXY STATEMENT

This Proxy Statement is furnished in connection with the solicitation of proxies to be voted at the 2024 Annual Meeting of Stockholders (the “Annual Meeting”) of REGENXBIO Inc., which will be held on May 31, 2024, at 9:00 a.m. local time at the Company’s offices located at 9804 Medical Center Drive, Rockville, Maryland 20850.

When this Proxy Statement refers to “REGENXBIO,” the “Company,” “we,” “us” or “our,” it is referring to REGENXBIO Inc.

We are making this Proxy Statement and our Annual Report on Form 10‑K for the fiscal year ended December 31, 2023 (the “Annual Report”) available to stockholders at www.proxyvote.com. On or about April 4, 2024, we will begin mailing to our stockholders a Notice of Internet Availability of Proxy Materials (the “Notice”) containing instructions on how to access and review this Proxy Statement and the Annual Report. The Notice also instructs you how you may submit your proxy over the internet or via telephone. If you would like to receive a printed copy of our proxy materials, you should follow the instructions for requesting those materials included in the Notice.