Offseason
- December 9, 1987: Phil Bradley and Tim Fortugno were traded by the Mariners to the Philadelphia Phillies for Glenn Wilson, Mike Jackson, and Dave Brundage (minors).
- December 21, 1987: John Moses was released by the Mariners.
- December 22, 1987: Lee Guetterman, Clay Parker, and Wade Taylor were traded by the Mariners to the New York Yankees for Steve Trout and Henry Cotto.
- January 19, 1988: John Rabb was signed as a free agent by the Mariners.
Read more about this topic: 1988 Seattle Mariners Season