REGENXBIO Inc.

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

(Amendment No. )

Filed by the Registrant☒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))

☒ Definitive Proxy Statement

☐ Definitive Additional Materials

☐ Soliciting Material Pursuant to §240.14a-12

(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):

☒ No fee required

☐ Fee paid previously with preliminary materials

☐ Fee computed on table in exhibit required by Item 25(b) per Exchange Act Rule 14a-6(i)(1) and 0-11

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 29, 2026, 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 Jean Bennett, M.D., Ph.D., A.N. “Jerry” Karabelas, Ph.D., and Daniel Tassé, to serve as Class II directors until the 2029 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, 2026;

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

To approve a stock option exchange for our non-executive employees;

To approve a stock option exchange for our executive employees; 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, 2026 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 29, 2026:

The Company’s Notice of Annual Meeting, Proxy Statement and Annual Report on Form 10‑K for the fiscal year ended December 31, 2025 are available atwww.proxyvote.com.

By Order of the Board of Directors,

Curran SimpsonPresident and Chief Executive Officer

Rockville, MarylandApril 14, 2026

This Proxy Statement is first being mailed to the stockholders of the Company on or about April 14, 2026.

TABLE OF CONTENTS

i

Rockville, MD 20850

PROXY STATEMENT

This Proxy Statement is furnished in connection with the solicitation of proxies to be voted at the 2026 Annual Meeting of Stockholders (the “Annual Meeting”) of REGENXBIO Inc., which will be held on May 29, 2026, 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, 2025 (the “Annual Report”) available to stockholders at www.proxyvote.com. On or about April 14, 2026, 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.