The 1990 Seattle Mariners season was the 14th for the Seattle Mariners in Major League Baseball. Under second-year manager Jim Lefebvre, they finished fifth in the American League West at 77–85 (.475). It was the second-best record in the M's short history; the win total was one behind the club record set in 1987. The Mariners hit six grand slams, the most in MLB in 1990.