Form DEFA14A Quanterix Corp

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.                            )

MMMMMMMMMMMM The Sample Company C 1234567890 000004 ENDORSEMENT_LINE______________ SACKPACK_____________ MR A SAMPLE DESIGNATION (IF ANY) ADD 1 ADD 2 ADD 3 ADD 4 ADD 5 ADD 6 Online Go to www.investorvote.com/QTRX or scan the QR code — login details are located in the shaded bar below. 2021 Stockholder Meeting Notice 1234 5678 9012 345 Important Notice Regarding the Availability of Proxy Materials for the Quanterix Corporation Stockholder Meeting to be held on June 24, 2021. Under Securities and Exchange Commission rules, you are receiving this notice that the proxy materials for the annual stockholders’ meeting are available on the Internet. Follow the instructions below to view the materials and vote online or request a copy. The items to be voted on at the virtual annual meeting are on the reverse side. Your vote is important! This communication presents only an overview of the more complete proxy materials that are available to you on the Internet. We encourage you to access and review all of the important information contained in the proxy materials before voting. The 2021 proxy statement and our 2020 annual report to stockholders are available for viewing, printing and downloading at: www.investorvote.com/QTRX Easy Online Access — View your proxy materials and vote. Step 1:Go to www.investorvote.com/QTRX. Step 2:Click on the icon on the right to view meeting materials. Step 3:Return to the investorvote.com window and follow the instructions on the screen to log in. Step 4:Make your selections as instructed on each screen for your delivery preferences. Step 5:Vote your shares. When you go online, you can also help the environment by consenting to receive electronic delivery of future materials. Obtaining a Copy of the Proxy Materials – If you want to receive a copy of the proxy materials, you must request one. There is no charge to you for requesting a copy. Please make your request as instructed on the reverse side on or before June 14, 2021 to facilitate timely delivery. MMMMMMMMM 03GF4B 2 N O T C O Y+

2021 Stockholder Meeting Notice Quanterix Corporation’s Annual Meeting of Stockholders will be held on June 24, 2021, at 10:00 a.m. Eastern Time, virtually via the internet at www.meetingcenter.io/255011196. To access the virtual meeting, you must have the 15 digit number that is printed in the shaded bar located on the reverse side of this form. The password for the meeting is QTRX2021. Proposals to be voted on at the meeting are listed below along with the Board of Directors’ recommendations. The Board of Directors recommends a vote FOR all the nominees listed and FOR Proposal 2: Election of Class I Directors for a three-year term expiring in 2024: 01 - Martin D. Madaus, Ph.D. 02 - Laurie J. Olson Ratification of the appointment of Ernst & Young LLP as the independent registered public accounting firm for the fiscal year ending December 31, 2021. PLEASE NOTE – YOU CANNOT VOTE BY RETURNING THIS NOTICE. To vote your shares you must go online or request a paper copy of the proxy materials to receive a proxy card. Here’s how to order a copy of the proxy materials and select delivery preferences: Current and future delivery requests can be submitted using the options below. If you request an email copy, you will receive an email with a link to the current meeting materials. PLEASE NOTE: You must use the number in the shaded bar on the reverse side when requesting a copy of the proxy materials. Internet – Go to www.investorvote.com/QTRX. Phone – Call us free of charge at 1-866-641-4276. Email – Send an email to [email protected] with “Proxy Materials Quanterix Corporation” in the subject line. Include your full name and address, plus the number located in the shaded bar on the reverse side, and state that you want a paper copy of the meeting materials. To facilitate timely delivery, requests for a paper copy of proxy materials must be received by June 14, 2021.

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