Company: RGNX
Filing Date: 2025-06-25
Form Type: 424B3
Source: 0001193125-25-146911
Chunk: 3

Company: REGENXBIO Inc.
Filing Date: 2025-06-25
Form: 424B3
Chunk 3
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 |     | our expectations regarding regulatory developments in the United States and foreign countries; |

ii

| • |     | the impact of any government-imposed tariffs or other trade barriers on cost of goods and services, particularly 
 related to partnered product candidates; and                                                                     |

| • |     | changes in the financial markets and banking system that may affect the availability and terms on which we may                                             
 obtain financing and our ability to accurately predict how long our existing cash resources will be sufficient to fund our anticipated operating expenses. |

You should carefully read the section of this prospectus titled “ Risk Factors,” as well as both the “ Risk Factors” and “ Management’s Discussion and Analysis of Financial Condition and Results of Operations” sections included in our quarterly and annual filings with the U.S. Securities and Exchange Commission (the “SEC”), that are incorporated herein by reference, for a discussion of the risks, uncertainties, assumptions and other important factors that could cause our actual results or developments to differ materially and adversely from those projected in the forward-looking statements. The actual results or developments anticipated may not be realized or, even if substantially realized, they may not have the expected consequences to or effects on us or our businesses or operations. Such statements are not guarantees of future performance, and actual results or developments may differ materially and adversely from those projected in the forward-looking statements. These forward-looking statements made by us in this prospectus, or in the information incorporated by reference herein, speak only as of the date of the respective document. Except as required by law, we disclaim any duty to update any forward-looking statements, whether as a result of new information, future events, or otherwise. iii

ABOUT THIS PROSPECTUS

This prospectus provides you with a general description of the Shares that may be resold by the Selling Stockholder. In certain circumstances,
we may provide a prospectus supplement or free writing prospectus that will contain specific information about the terms of a particular offering by the Selling Stockholder. We also may provide a prospectus supplement to add, update or change
information contained in this prospectus. To the extent there is a conflict between the information contained in this prospectus and in any prospectus supplement or free writing prospectus, you should rely on the information in the prospectus
supplement or free writing prospectus, provided that if any statement in one of these documents is inconsistent with a statement in another document having a later date—for example, a document incorporated by reference in this prospectus or any
prospectus supplement—the