During the 1975 regular season, Petrocelli batted .239 in 115 games, with seven home runs and 59 RBIs. He missed the second half of August due to an inner ear problem, likely related to his 1974 beaning. Boston won the American League East, and faced the Oakland Athletics in the American League Championship Series. Petrocelli started all three games at third base, as the Red Sox swept the A's, while batting 2-for-12 (.167) with a home run and two RBIs. In the 1975 World Series, which Boston lost to the Cincinnati Reds, Petrocelli hit .308 (8-for-26) with four RBIs, starting all seven games at third base. He did not commit an error during the 1975 postseason.