Company: RGNX
Filing Date: 2025-11-26
Form Type: S-3
Source: 0001193125-25-300539
Chunk: 2

Company: REGENXBIO Inc.
Filing Date: 2025-11-26
Form: S-3
Chunk 2
---

Units

We may offer and sell, from time to time, in one or more offerings, up to $300,000,000 of our (i) shares of common stock,
(ii) shares of preferred stock, (iii) warrants to purchase common stock, preferred stock or debt securities, (iv) debt securities, (v) rights to purchase common stock, preferred stock, debt securities or units and (vi) units
that include any of these securities. We may sell any combination of these securities in one or more offerings in amounts, at prices and on terms that we will determine at the time of the offering.

This prospectus provides you with a general description of the securities we may offer. Each time we offer securities pursuant to this
prospectus, we will provide a prospectus supplement containing specific terms of the particular offering together with this prospectus. You should read this prospectus and the applicable prospectus supplement carefully before you invest in any
securities. The prospectus supplement also may add, update or change information contained in this prospectus. This prospectus may not be used to offer and sell securities unless accompanied by the applicable prospectus supplement.

The securities may be offered and sold in the same offering or separate offerings directly or to or through underwriters or dealers, and also
to other purchasers or through agents. The names of any underwriters or agents that are included in a sale of securities to you, and any applicable commissions or discounts, will be stated in the applicable prospectus supplement.

Our common stock is listed on The Nasdaq Global Select Market under the symbol “RGNX.” On November 25, 2025, the closing
price of our common stock was $13.45.

Investing in our securities involves significant risks. Before buying our securities, you should carefully read and consider the risks we describe in this prospectus and in any accompanying prospectus supplement, as well as the risk factors that are incorporated by reference into this prospectus and in any accompanying prospectus supplement from our filings made with the Securities and Exchange Commission. See “ Risk Factors” beginning on page 3 of this prospectus.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

The date of this prospectus is , 2025

TABLE OF CONTENTS

| SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS |     | ii |
| ABOUT THIS