Amendment No. 3 to Harvard Bioscience, Inc. Employee Stock Purchase Plan, effective as of May 18, 2017

EXHIBIT 10.51

AMENDMENT NO. 3 TO

HARVARD BIOSCIENCE, INC.

EMPLOYEE STOCK PURCHASE PLAN

This Amendment No. 3 to the Harvard Bioscience, Inc. Employee Stock Purchase Plan (the “Plan”) is effective as of May 18, 2017 (the “Effective Date”).

In accordance with Section 18 of the Plan, as approved by the stockholders of Harvard Bioscience, Inc. on the Effective Date, in order to increase the number of shares of common stock reserved for issuance under the Plan to One Million Fifty Thousand (1,050,000), the Plan is hereby amended as follows, effective as of the Effective Date:

“DATE AMENDMENT NO. 1 TO PLAN APPROVED BY BOARD OF DIRECTORS: AUGUST 2, 2011.

DATE AMENDMENT NO. 2 TO PLAN APPROVED BY BOARD OF DIRECTORS: FEBRUARY 26, 2013.

DATE AMENDMENT NO. 2 TO PLAN APPROVED BY STOCKHOLDERS: MAY 23, 2013.

DATE AMENDMENT NO. 3 TO PLAN APPROVED BY BOARD OF DIRECTORS: MARCH 31, 2017.

DATE AMENDMENT NO. 3 TO PLAN APPROVED BY STOCKHOLDERS: MAY 18, 2017.”

IN WITNESS WHEREOF, the Harvard Bioscience, Inc. has duly executed this amendment to be effective as the date first above written.