RGEN Repligen Corp

Free Writing Prospectus

Filed pursuant to Rule 433

Relating to the

Preliminary Prospectus Supplements each dated July 15, 2019 to the

Prospectus dated April 29, 2019

Registration No. 333-231098

Pricing Term Sheet

dated as of July 16, 2019

Repligen Corporation

Concurrent Offerings of

$250,000,000 Aggregate Principal Amount of 0.375% Convertible Senior Notes due 2024

(the “Convertible Notes Offering”)

and

1,380,000 Shares of Common Stock, par value $0.01 per Share

(the “Common Stock Offering”)

The information in this pricing term sheet relates only to the Convertible Notes Offering and the Common Stock Offering (together, the “Offerings”) and should be read together with (i) the preliminary prospectus supplement dated July 15, 2019 relating to the Convertible Notes Offering (the “Convertible Notes Preliminary Prospectus Supplement”), including the documents incorporated by reference therein and (ii) the preliminary prospectus supplement dated July 15, 2019 relating to the Common Stock Offering (the “Common Stock Preliminary Prospectus Supplement” and, together with the Convertible Notes Preliminary Prospectus Supplement, the “Preliminary Prospectus Supplements”), including the documents incorporated by reference therein. Neither the Convertible Notes Offering nor the Common Stock Offering is contingent on the successful completion of the other offering. The information in this pricing term sheet supersedes the information in the Convertible Notes Preliminary Prospectus Supplement and the Common Stock Preliminary Prospectus Supplement to the extent inconsistent with the information therein. In all other respects, this pricing term sheet is qualified in its entirety by reference to the Convertible Notes Preliminary Prospectus Supplement and the Common Stock Preliminary Prospectus Supplement, as the case may be. The Issuer has increased the size of the Common Stock Offering to $120,060,000 (or $138,069,000 if the underwriters’ option to purchase additional Notes is exercised in full). Each final prospectus supplement for the Offerings will reflect conforming changes relating to such increase in the size of the Common Stock Offering.

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The Issuer has filed a registration statement (including a prospectus and related preliminary prospectus supplements for each of the Offerings) with the U.S. Securities and Exchange Commission (the “SEC”) for the Offerings to which this communication relates. Before you invest, you should read the Convertible Notes Preliminary Prospectus Supplement or the Common Stock Preliminary Prospectus Supplement, as applicable, the related base prospectus and the other documents the Issuer has filed with the SEC that are incorporated by reference into the Convertible Notes Preliminary Prospectus Supplement or the Common Stock Preliminary Prospectus Supplement, as applicable, for more complete information about the Issuer, the Convertible Notes Offering and the Common Stock Offering. You may get these documents for free by visiting EDGAR on the SEC’s website at http://www.sec.gov. Alternatively, copies may be obtained from J.P. Morgan Securities LLC, Attention: Broadridge Financial Solutions, 1155 Long Island Avenue, Edgewood, NY 11717, or by telephone at (866) 803-9204, or by email at prospectus-eq_fi@jpmchase.com, from Stephens Inc., Attention: Equity Syndicate Desk, 111 Center Street, Little Rock, AR 72201, or by telephone at (800) 643-9691, or by email at prospectus@stephens.com or from Janney Montgomery Scott, Attention: Equity Syndicate, 60 State Street, 35th Floor, Boston, MA 02109, or by telephone at (617) 557-2971, or by email at prospectus@janney.com. You may also access the Convertible Notes Preliminary Prospectus Supplement and the related base prospectus by clicking on the following link: https://www.sec.gov/Archives/edgar/data/730272/000119312519194004/d777344d424b5.htm, and the Common Stock Preliminary Prospectus Supplement and the related base prospectus by clicking on the following link: https://www.sec.gov/Archives/edgar/data/730272/000119312519193992/d772731d424b5.htm.

ANY DISCLAIMERS OR OTHER NOTICES THAT MAY APPEAR BELOW ARE NOT APPLICABLE TO THIS COMMUNICATION AND SHOULD BE DISREGARDED. SUCH DISCLAIMERS OR OTHER NOTICES WERE AUTOMATICALLY GENERATED AS A RESULT OF THIS COMMUNICATION BEING SENT VIA BLOOMBERG OR ANOTHER EMAIL SYSTEM.

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