Major League Baseball (MLB) is a professional baseball organization and the oldest major professional sports league in the world. As of 2022, a total of 30 teams play in Major League Baseball15 teams in the National League (NL) and 15 in the American League (AL)with 29 in the United States and 1 in Canada. The NL and AL were formed in 1876 and 1901, respectively. Beginning in 1903, the two leagues signed the National Agreement and cooperated but 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.

Baseball's first all-professional team, the Cincinnati Red Stockings, was founded in 1869. Before that, some teams had secretly paid certain players. The first few decades of professional baseball were characterized by rivalries between leagues and by players who 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 in the United States survived the Black Sox Scandal, a conspiracy to fix the 1919 World Series. The sport rose in popularity in the 1920s and survived potential downturns during the Great Depression and World War II. Shortly after the war, Jackie Robinson broke baseball's color barrier.

The 1950s and 1960s were a time of club expansion and relocation for the AL and NL. Modern stadiums with artificial turf surfaces began to change the game in the 1970s and 1980s. Home runs dominated the game during the 1990s, and media reports disclosed the use of anabolic steroids among MLB players in the mid-2000s. In 2006, an investigation produced the Mitchell Report, which implicated many players in the use of performance-enhancing substances, including at least one player from each team.

Each team plays 162 games per each 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 that dates to 1903. 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 with more than 69.6 million spectators in 2018.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.

MLB is the second-wealthiest professional sport league by revenue after the National Football League (NFL).

The National League of Professional Baseball Clubs, known simply as the National League (NL), is the older of two leagues constituting Major League Baseball (MLB) in the United States and Canada, and the world's oldest extant professional team sports league. Founded on February 2, 1876, to replace the National Association of Professional Base Ball Players (NAPBBP) of 1871–1875 (often called simply the "National Association"), the NL is sometimes called the Senior Circuit, in contrast to MLB's other league, the American League, which was founded 25 years later and is called the "Junior Circuit".

Both leagues currently have 15 teams. After two years of conflict in a "baseball war" of 1901–1902, the two eight-team leagues agreed in a "peace pact" to recognize each other as "major leagues", draft rules regarding player contracts, prohibiting "raiding", regulating relationships with minor leagues and lower level clubs, and with each establishing a team in the nation's largest metropolis of New York City, and the league champions of 1903 arranged to compete against each other in the new professional baseball championship tournament with the inaugural "World Series" that Fall of 1903, succeeding earlier similar national series in previous decades since the 1880s. After the 1904 champions failed to reach a similar agreement, the two leagues also formalized the new World Series tournament beginning in 1905 as an arrangement between the leagues themselves. National League teams have won 51 of the 117 World Series championships contested from 1903 to 2021.