The Mariners selected Mastrobuoni's contract from Triple-A Tacoma on Friday.
Mastrobuoni was DFA'd by the Mariners in mid-June but is back with the big club to provide depth in the outfield after Julio Rodriguez (concussion) was placed on the 7-day injured list. Mastrobuoni has spent most of the 2026 season in Triple-A, where he has a .603 OPS with five steals, one home run and three RBI over 71 plate appearances.