In Major League Baseball, the American League Championship Series (ALCS), played in October, is a playoff round that determines the winner of the American League pennant. The winner of the series advances to play the winner of the National League Championship Series in baseball's championship, the World Series. It began in 1969, when the American League was reorganized into two divisions, East and West. The winners of each division played each other in a best-of-five series to determine who would advance to the World Series. In 1985, the format changed to best-of-seven.

In 1994, the league was restructured into three divisions, with the three division winners and a wild-card team advancing to a best-of-five playoff round, known as the American League Division Series or ALDS. The winners of that round then advanced to the best-of-seven ALCS. This is the system currently in use.

The ALCS and NLCS, since the expansion to best-of-seven, are always played in a 2-3-2 format: Games 1, 2, 6 * , and 7 * are played in the stadium of the team that has home field advantage, and Games 3, 4, and 5 * are played in the stadium of the team that does not. ( * if necessary. The series concludes when one team records its fourth win.) Home field advantage is given to the team that has the better record, unless that team happens to be the Wild Card team. In that case, the other team gets home field advantage, because by rule the Wild Card team is never allowed home field advantage in a Division Series or LCS. In the event that both teams have identical records in the regular season, home field advantage goes to the team that has the winning head-to-head record.

The Tampa Bay Rays and Texas Rangers are the only American League teams to never appear in an ALCS. The other 12 teams and the Milwaukee Brewers (members of the AL from 1969-1997) have each appeared at least once.

A Most Valuable Player (MVP) award is given to the outstanding player in the ALCS. No MVP award is given for Division Series play. The William Harridge Trophy is awarded to the ALCS winner.

For American League pennant winners prior to 1969, see American League pennant winners 1901-68.