The 2021 American League Championship Series was the best-of-seven series in Major League Baseball (MLB) between the Boston Red Sox and the Houston Astros. The series determined the American League pennant winner, the Astros, who advanced to the 2021 World Series. The series was played in a 2–3–2 format, with the first two and last two (if game 7 had been necessary) games played at the home ballpark of the higher seeded team, the Astros. Fox and FS1 televised all games in the United States. The Astros would go on to lose to the Atlanta Braves in the World Series in six games.