The 1975–76 Boston Celtics season was their 30th in the National Basketball Association (NBA) and concluded with their 13th championship, defeating the Phoenix Suns in six games in the 1976 NBA Finals to win their 13th NBA Championship . The Celtics also won their division for the 5th consecutive season, and made their 14th finals appearance.

The Celtics lost Don Chaney to the American Basketball Association before the 1975–76 season. To fill the gap in the backcourt they traded Paul Westphal to the Phoenix Suns for Charlie Scott, who had averaged more than 20 points in each of the previous three seasons. Despite an uncharacteristically weak bench, the Celtics finished in first place in their division and the second best record in the NBA this season. Boston earned a shot at another NBA title by defeating the Buffalo Braves and then the Cleveland Cavaliers in the playoffs.

Boston's opponents in the 1976 NBA Finals were the Phoenix Suns, who had posted a 42–40 regular-season record. The Team in Green was the oddsmakers' choice in the contest. The Celtics took the first two games at Boston Garden, but the Suns came back to win games 3 and 4 on their home court. Game 5 ranks among the all-time thrillers in NBA history. The Suns trailed by 5 points with less than a minute left on the clock, but Westphal made up the deficit almost single-handedly, sending the game into a first overtime period, which ended in a tie.

John Havlicek's basket with 2 seconds remaining in double overtime gave the Celtics a one-point lead, which Boston stretched to two points after sinking a technical foul. Then the Suns' Garfield Heard hit a last-second basket to send the contest into a third overtime. The longest game in NBA history finally ended, after three extra periods, with the Celtics winning 128–126. Two days later Boston captured yet another NBA championship, the 13th in franchise history.