EXHIBIT NO. 99.(a) 27

MFS SERIES TRUST X

CERTIFICATION OF AMENDMENT

TO THE DECLARATION OF TRUST

ESTABLISHMENT AND DESIGNATION

OF CLASSES

Pursuant to Sections 6.10 and 9.3 of the Amended and Restated Declaration of Trust dated December 16, 2004, as amended (the “Declaration”) of MFS Series Trust X, a business trust organized under the laws of The Commonwealth of Massachusetts (the “Trust”), the undersigned Trustees of the Trust, being a majority of the Trustees of the Trust, do hereby divide the shares of MFS Blended Research Small Cap Equity Fund (the “Fund”), a series of the Trust, to create additional classes of shares, within the meaning of Section 6.10, as follows:

1.The additional classes of Shares are designated “Class 529A Shares,” “Class 529B Shares,” and “Class 529C Shares”;

2.Class 529A Shares, Class 529B Shares, and Class 529C Shares shall be entitled to all the rights and preferences accorded to shares under the Declaration;

3.The purchase price of Class 529A Shares, Class 529B Shares, and Class 529C Shares, the method of determination of the net asset value of Class 529A Shares, Class 529B Shares, and Class 529C Shares, the price, terms and manner of redemption of Class 529A Shares, Class 529B Shares, and Class 529C Shares, any conversion feature of Class 529A Shares, Class 529B Shares, and Class 529C Shares, and relative dividend rights of holders of Class 529A Shares, Class 529B Shares, and Class 529C Shares shall be established by the Trustees of the Trust in accordance with the Declaration and shall be set forth in the current prospectus and statement of additional information of the Trust or any series thereof, as amended from time to time, contained in the Trust’s registration statement under the Securities Act of 1933, as amended;

4.All shares shall vote together as a single class except that shares of a class may vote separately on matters affecting only that class and shares of a class not affected by a matter will not vote on that matter; and

5.A class of shares of any series of the Trust may be terminated by the Trustees by written notice to the Shareholders of the class.

Pursuant to Section 6.11 of the Declaration, this instrument shall be effective upon the execution by a majority of the Trustees of the Trust.

IN WITNESS WHEREOF, a majority of the Trustees of the Trust have executed this amendment, in one or more counterparts, all constituting a single instrument, as an instrument under seal in The Commonwealth of Massachusetts, as of April 8, 2016 and further certify, as provided by the provisions of Section 9.3(c) of the Declaration, that this amendment was duly adopted by the undersigned in accordance with Section 9.3(a) of the Declaration.

STEVEN E. BULLER	JOHN P. KAVANAUGH
Steven E. Buller	John P. Kavanaugh
c/o MFS Investment Management	c/o MFS Investment Management
111 Huntington Avenue	111 Huntington Avenue
Boston, MA 02199	Boston, MA 02199
ROBERT E. BUTLER	ROBERT J. MANNING
Robert E. Butler	Robert J. Manning
c/o MFS Investment Management	MFS Investment Management
111 Huntington Avenue	111 Huntington Avenue
Boston, MA 02199	Boston, MA 02199
MAUREEN R. GOLDFARB	MARYANNE L. ROEPKE
Maureen R. Goldfarb	Maryanne L. Roepke
c/o MFS Investment Management	c/o MFS Investment Management
111 Huntington Avenue	111 Huntington Avenue
Boston, MA 02199	Boston, MA 02199
DAVID H. GUNNING	ROBIN A. STELMACH
David H. Gunning	Robin A. Stelmach
c/o MFS Investment Management	MFS Investment Management
111 Huntington Avenue	111 Huntington Avenue
Boston, MA 02199	Boston, MA 02199
WILLIAM R. GUTOW	LAURIE J. THOMSEN
William R. Gutow	Laurie J. Thomsen
c/o MFS Investment Management	c/o MFS Investment Management
111 Huntington Avenue	111 Huntington Avenue
Boston, MA 02199	Boston, MA 02199

MICHAEL HEGARTY	ROBERT W. UEK
Michael Hegarty	Robert W. Uek
c/o MFS Investment Management	c/o MFS Investment Management
111 Huntington Avenue	111 Huntington Avenue
Boston, MA 02199	Boston, MA 02199