Major League Baseball (MLB) is a professional baseball organization and the oldest major professional sports league in the world. MLB is composed of 30 teams, divided equally between the National League (NL) and the American League (AL), with 29 in the United States and 1 in Canada. Formed in 1876 and 1901 respectively, the NL and AL cemented their cooperation with the National Agreement in 1903. They remained legally separate entities until 2000, when they merged into a single organization led by the Commissioner of Baseball. MLB is headquartered in Midtown Manhattan. It is considered one of the major professional sports leagues in the United States and Canada.

Baseball's first all-professional team, the Cincinnati Red Stockings, was founded in 1869. The first few decades of professional baseball saw rivalries between leagues, and players often jumped from one team or league to another. The period before 1920 was the dead-ball era, when home runs were rarely hit. Professional baseball survived the Black Sox Scandal, a conspiracy to fix the 1919 World Series, then rose in popularity in the following decade. It survived the Great Depression and World War II. Shortly after the war, Jackie Robinson broke baseball's color barrier.

The 1950s and 1960s saw the AL and NL add clubs; some moved around the country. Modern stadiums with artificial turf surfaces began to change the game in the 1970s and 1980s. Home runs dominated the game during the 1990s. In the mid-2000s, media reports disclosed the use of anabolic steroids among MLB players; a 2006-07 investigation produced the Mitchell Report, which found that many players had used steroids and other performance-enhancing substances, including at least one player from each team.

Each team plays 162 games per season, and six teams in each league advance to a four-round postseason tournament that culminates in the World Series, a best-of-seven championship series between the two league champions first played in 1903. The New York Yankees have the most championships: 27. The reigning champions are the Houston Astros, who defeated the Philadelphia Phillies, 4–2, in the 2022 World Series.

MLB is the second-wealthiest professional sport league by revenue after the National Football League (NFL). Baseball games are broadcast on television, radio, and the internet throughout North America and in several other countries. MLB has the highest total season attendance of any sports league in the world; in 2018, it drew more than 69.6 million spectators.

MLB also oversees Minor League Baseball, which comprises lower-tier teams affiliated with the major league clubs. MLB and the World Baseball Softball Confederation jointly manage the international World Baseball Classic tournament.