Apr. 10, 2017

MFS SERIES TRUST IX | MFS® Inflation-Adjusted Bond Fund

MFS® Inflation-Adjusted Bond Fund

SUPPLEMENT TO THE PROSPECTUS

The date of this supplement is April 10, 2017.

For Certain MFS® Funds

Effective immediately, the following sentence is added to the first paragraph in the sub-section entitled “Fees and Expenses” under the main heading entitled “Summary of Key Information”:

Investors may also pay commissions or other fees to their financial intermediary when they buy and hold shares of the fund, which are not reflected below.

1028417 MULTI-SUP-III-041017