Mariners catcher Nick Raposo was designated for assignment to make room for Mastrobuoni on the 40-man roster.
Mastrobuoni slashed .194/.245/.224/.470 in 50 games for the Cubs last season. Despite not being anything to write home about as a hitter, the Mariners no doubt made the move for Mastrobuoni's glove.
Last season, the three-year vet played 17 games at third base, 10 at both shortstop and second base, two in right field, and one in left field.
Mastrobuoni is a former 14th-round pick of the 2016 MLB Draft. He made his debut in 2022 with the Tampa Bay Rays before finding a spot on the Cubs roster in 2023, in which he played 60 games hitting .241.
Mastrobuoni also played for Team Italy in the latest World Baseball Classic.