The 1903 major league baseball season began on April 16, 1903. The regular season ended on September 29, with the Pittsburgh Pirates and the Boston Americans as regular season champions of the National League and American League, respectively. The postseason began with Game 1 of the first modern World Series on October 1 and ended with Game 8 on October 13. The Americans defeated the Pirates, five games to three, capturing their first championship in franchise history.
The 1903 season saw the return of a postseason championship series, the World Series, following the one-off 1900 Chronicle-Telegraph Cup. It was also the first inter-league series since the 1890 World's Championship Series between the National League and defunct-since- American Association. The 1903 World Series would also mark the first championship series that is still celebrated today, as all previous series are considered pre-modern and these early contests are discussed by MLB and baseball historians separately. However, the 1903 arrangement was primarily between the two participating clubs rather than a formal arrangement between the leagues, essentially making this season's World Series a voluntary event.
The defunct Baltimore Orioles were replaced by a new franchise in New York City known as the New York Highlanders; it was the last change to the lineup of AL and NL franchises until . The American League would not return to Baltimore until the St. Louis Browns relocated their as a new Baltimore Orioles in . The Chicago Orphans and Cleveland Bronchos were renamed as the Chicago Cubs and Cleveland Naps, respectively.
The 1903 schedule consisted of 140 games for all teams in the American League and National League, each of which had eight teams. Each team was scheduled to play 20 games against the other seven teams of their respective league. This continued the format put in place for the season. This would be the last season with this format, as the following season would see an increase of games played.
National League Opening Day took place on April 16 with four teams playing, while American League Opening Day did not take place until April 20, with a doubleheader between the Philadelphia Athletics and the Boston Americans. The National League would see its final day of the regular season on September 27, while the American League would see its final day of the season on September 29. The inaugural World Series took place between October 1 and October 13.
The 1903 season saw the following rule changes:
An asterisk (*) denotes the ballpark a team played the minority of their home games at
Blue laws restricted Sunday activities in several localities, causing several teams to play at ballparks in a different locality.
15 tie games (6 in AL, 9 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 13 with the Boston Americans defeating the Pittsburgh Pirates in the 1903 World Series in eight games.
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Termed the "Battle of Ohio", the Cincinnati Reds and Cleveland Naps played an unofficial best of 11-game exhibition series after the regular season, with Cleveland winning the series six games to three.
The 1903 season saw the enfranchisement of the New York Highlanders, in place of the folded Baltimore Orioles, playing at Hilltop Park in New York City, New York, where they would play for ten seasons through .
Regarding games that were rescheduled to Sunday, and existing blue laws:
On August 8, in what is known as the worst disaster in American sports spectating history, Black Saturday, a section of balcony at the Philadelphia Phillies' home at the Baker Bowl collapsed, killing 12 spectators and injuring 232. The Philadelphia Phillies played their last 16 of their 61 home games at the home of the American League crosstown Philadelphia Athletics, Columbia Park from August 20.