Major League Soccer (MLS) is a men's professional soccer league sanctioned by the United States Soccer Federation, which represents the sport's highest level in the United States. The league comprises 28 teams—25 in the U.S. and 3 in Canada—and will expand to 29 teams for the 2023 season. The league is headquartered in Midtown Manhattan.

Major League Soccer is the most recent in a series of men's premier professional national soccer leagues established in the United States and Canada. The predecessor of MLS was the North American Soccer League (NASL), which existed from 1968 until 1984. MLS was founded in 1993 as part of the United States' successful bid to host the 1994 FIFA World Cup. The inaugural season took place in 1996 with ten teams. MLS experienced financial and operational struggles in its first few years, losing millions of dollars and folding two teams in 2002. Since then, developments such as the proliferation of soccer-specific stadiums around the league, implementation of the Designated Player Rule allowing teams to sign star players such as David Beckham, and national TV contracts have made MLS profitable. With an average attendance of over 20,000 per game, MLS has the fourth-highest average attendance of any professional sports league in the U.S. and Canada after the National Football League (NFL) and Major League Baseball (MLB), and Canadian Football League (CFL) and was the seventh-highest attended professional soccer league worldwide by 2013.

The MLS regular season typically starts in late February or early March and runs through mid-October, with each team playing 34 games; the team with the best record is awarded the Supporters' Shield. Fourteen teams compete in the postseason MLS Cup Playoffs in late October and November, culminating in the league's championship game, the MLS Cup.

Instead of operating as an association of independently owned clubs, MLS is a single entity in which each team is owned by the league and individually operated by the league's investors. The league has a fixed membership like most sports leagues in the United States and Canada, which makes it one of the few soccer leagues that does not use a promotion and relegation process.