The 1978 Seattle Mariners season was the second in franchise history. The Mariners ended the season by finishing last in the American League West with a record of 56–104 (.350), their first last place finish and six games worse than their inaugural season.

Note: Pos = Position; G = Games played; AB = At bats; H = Hits; Avg. = Batting average; HR = Home runs; RBI = Runs batted in

Note: G = Games played; AB = At bats; H = Hits; Avg. = Batting average; HR = Home runs; RBI = Runs batted in

Note: G = Games pitched; IP = Innings pitched; W = Wins; L = Losses; ERA = Earned run average; SO = Strikeouts

Note: G = Games pitched; IP = Innings pitched; W = Wins; L = Losses; ERA = Earned run average; SO = Strikeouts

Note: G = Games pitched; W = Wins; L = Losses; SV = Saves; ERA = Earned run average; SO = Strikeouts

After fielding only one minor league affiliate in 1977, the Mariners added a Triple-A and Single-A team in 1978.