Before the start of the 1978 season, Stein re-signed with the Seattle Mariners. His contract meant he was now signed through the 1980 season. In May 1978, he bruised his left hand, which caused him to miss some playing time. On August 25, he broke up Dennis Martínez's potential no-hitter in the seventh inning of a game against the Baltimore Orioles. On August 28, in a game against the Boston Red Sox, Stein had a season-high four hits. On the season, he batted .261 with 41 runs scored, 105 hits, 24 doubles, four triples, four home runs, and 37 RBIs in 114 games played. In the field, Stein played 67 games at third base, 17 games at second base, and three games at shortstop. His 24 errors at third base was second in the American League.