The Arizona Complex League (ACL) is a rookie-level Minor League Baseball league that operates in and around Phoenix, Arizona, since 1988. Prior to 2021, it was known as the Arizona League (AZL). Along with the Florida Complex League (FCL), it forms the lowest rung on the North American minor-league ladder.

ACL teams play at the minor league spring training complexes of their parent Major League Baseball (MLB) clubs and are owned by those parent clubs. Admission is not charged and no concessions are operated at the teams' games. Every Cactus League team fields at least one team in the league. Night games are commonly played in the spring training stadium, although games may also be played at the team's practice fields.

As of the 2021 season, there is no league limit to how many players can be on an active roster, but no team can have more than three players with four or more years of minor-league experience. Major-league players on rehabilitation assignments may also appear in the league.

The league began play in 1988 as the Arizona League (AZL) on an experimental basis with four teams playing a 60-game schedule—the teams were affiliated with the Milwaukee Brewers, Oakland Athletics, San Diego Padres, and a cooperative between the Boston Red Sox and Seattle Mariners. Games were scheduled in the morning in order to make sure the league did not compete with the Phoenix Firebirds of the Triple-A Pacific Coast League.

The Phoenix Diamondbacks debuted in 1996, the first affiliate of the Arizona Diamondbacks to start playing professional baseball, two years before their parent club joined Major League Baseball. A crowd of 6,124 attended the Diamondbacks' first game on June 25, 1996, a 15–7 loss to the Phoenix Athletics. A number of local dignitaries were at the game, including Buck Showalter and Jerry Colangelo.

The Colorado Rockies started their rookie league team a year before starting major league play as well in 1992, sharing a team with the Cubs. The team, composed of the Rockies' first-ever draft picks, received a different level of notoriety than the Diamondbacks for their first rookie league game: as the players wore either Rockies jerseys or Cubs jerseys, the team did not have their uniform pants ready at the start of the season, and had to borrow pants from the Cubs.

Between 1998 and 2000 an unaffiliated team composed of young players from the Mexican League played out of the Tucson Electric Park spring training complex, in part to add a fourth team to a southern division. Three Tucson-based teams (the Mexican All-Stars, the Diamondbacks, and the Rockies) would leave the league after the 2000 season, largely due to travel concerns, followed by the White Sox two years later. All three teams later relocated their spring training complexes to the Phoenix metropolitan area, and the Diamondbacks and White Sox reinstated their rookie league teams.

The start of the 2020 season was postponed due to the COVID-19 pandemic before ultimately being cancelled on June 30.