In the 1989 American League Championship Series, Stewart won both of the games he pitched with a 2.81 ERA. In the World Series, Stewart pitched a complete game shutout in the first game against the San Francisco Giants. The series was interrupted when an earthquake struck the Bay Area just prior to the start of Game 3. When the series resumed, 10 days later, Stewart started game three for the A's, and picked up his second win of the series when he allowed three runs in seven innings. The A's swept the series in four games and Stewart was selected as the MVP of the series going 2–0 with a 1.69 ERA. Stewart signed a new two-year $7 million contract extension with the Athletics after the season. This was the largest contract in baseball at the time.