The 1931 major league baseball season began on April 14, 1931. The regular season ended on September 27, with the St. Louis Cardinals and Philadelphia Athletics as the regular season champions of the National League and American League, respectively. The postseason began with Game 1 of the 28th World Series on October 1 and ended with Game 7 on October 10. In the second iteration of this World Series matchup (and a rematch of the previous year), the Cardinals defeated the Athletics, four games to three, capturing their second championship in franchise history, since their previous in . Going into the season, the defending World Series champions were the Philadelphia Athletics from the season.
This was the first season that the Baseball Writers' Association of America (BBWAA) selected a Most Valuable Player in each league.
The 1931 schedule consisted of 154 games for all teams in the American League and National League, each of which had eight teams. Each team was scheduled to play 22 games against the other seven teams of their respective league. This continued the format put in place since the season (except for ) and would be used until in the American League and in the National League.
Opening Day, April 14, featured all sixteen teams, the first time since the season. The final day of the regular season was on September 27, which also saw all sixteen teams play on the final day continuing the trend which began the previous season. This was the first time that both Opening Day and the final day of the season saw all sixteen teams play. The World Series took place between October 1 and October 10.
The 1931 season saw the following rule changes:
An asterisk (*) denotes the ballpark a team played the minority of their home games at
9 tie games (5 in AL, 4 in NL), which are not factored into winning percentage or games behind (and were often replayed again) occurred throughout the season.
The postseason began on October 1 and ended on October 10 with the St. Louis Cardinals defeating the Philadelphia Athletics in the 1931 World Series in seven games.
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<sup>1</sup> American League Triple Crown pitching winner
Across 80 homes games, the Boston Red Sox played all 15 of their Sunday games at the Boston Braves home field of Braves Field (the remaining 65 home games were played at Fenway Park). This was the last of three consecutive seasons playing all Sunday games at Braves Field and 3rd of four consecutive season playing some games at Braves Field.
This was the last of 31 consecutive seasons that the Cleveland Indians played at their inaugural field, League Park (they would play full seasons at the Park in and , while , and to saw home games split with Cleveland Stadium.