The American League of Professional Baseball Clubs, known simply as the American League (AL), is the younger of two leagues constituting Major League Baseball (MLB) in the United States and Canada. It developed from the Western League, a minor league based in the Great Lakes states, which eventually aspired to major league status. It is sometimes called the Junior Circuit because it claimed Major League status for the 1901 season, 25 years after the formation of the National League (the "Senior Circuit").

Since 1903, the American League champion has played in the World Series against the National League champion with only two exceptions: 1904, when the NL champion New York Giants refused to play their AL counterpart, and 1994, when a players' strike resulted in the cancellation of the Series. Through 2025, American League teams have won 68 of the 121 World Series played since 1903. The New York Yankees have won 27 World Series and 41 American League titles, the most in major league history. The Philadelphia/Kansas City/Oakland Athletics have won the second most AL titles with 15, followed by the Boston Red Sox with 14.

For decades, Major League baseball clubs only played teams from their own league during the regular season and playoffs, with only the two pennant winners meeting in the World Series. The AL and NL also employed and trained their own umpires, who only worked regular season games in their own league. These differences resulted in the leagues developing slightly different strategies and styles of play. The American League was usually regarded as the less "traditional" league during the 20th century, a reputation most exemplified by the introduction of the designated hitter rule in 1973, which encouraged AL managers to largely abandon "smallball" tactics. However, with the advent of free agency in the 1970s allowing for more player movement between leagues, the introduction of regular season interleague play in 1997, umpires working games across MLB beginning in 1999, and the NL's adoption of the designated hitter rule in 2022, the difference in play between the two major leagues has diminished considerably.

Though both leagues agreed to be jointly governed by a commissioner in 1920, they remained separate business entities with their own presidents and management. This was the case until after the 1999 season, when the American League legally merged with the National League under the auspices of Major League Baseball, which now operates much like other North American professional sports leagues, albeit with two "leagues" instead of "conferences".

Originally a minor league known as the Western League, which existed from 1885 to 1899 with teams in mostly Great Lakes states, the league changed its name to the American League for the 1900 season and the next year developed into a second major league as a competitor to the older National League. This was prompted by the NL dropping four teams following the 1899 season after having absorbed its previous rival, the American Association, which disbanded in 1891 after ten seasons.

In its early history of the late 1880s, the minor Western League struggled until 1894, when Ban Johnson became the president of the league. Johnson pushed the league to rise to major league status, after the name change to the American League was decided in a league meeting in Milwaukee, Wisconsin, at the former Republican Hotel. A historical marker is at the intersection of North Dr. Martin Luther King Jr. Drive and West Kilbourn Avenue where the hotel once stood.

In March 1904, Johnson moved the league's headquarters from Chicago to New York.

Babe Ruth, noted as one of the most prolific hitters in Major League Baseball history, spent the majority of his career in the American League with the Boston Red Sox and the New York Yankees. From 1973 to 2022 The American League had one notable difference versus the rival National League, as it had the designated hitter rule. Under the rule, a team may use a batter in its lineup who is not in the field defensively, replacing the pitcher in the batting order, compared to the old rule that made it mandatory for the pitcher to bat. In the last two decades, the season schedule has allowed occasional interleague play. In 1969, the AL (and NL) were divided into East and West divisions, with a postseason playoff series for the pennant and the right to play in the World Series.