The Mariners selected Montes's contract from Triple-A Tacoma on Tuesday.
The Mariners will give Montes a look in September as they try to make up ground in the American League Wild Card race. Montes has popped 38 home runs while collecting a .928 OPS between Double-A Arkansas and Tacoma this season, though it's also come with a 30.6 percent strikeout rate. The 21-year-old slugger should see regular playing time against righties, which would likely eat into the opportunities of the struggling Taylor Ward (.378 OPS with Seattle). Montes is starting at designated hitter and batting seventh Tuesday in Boston in his major-league debut.