The 2011–12 Boston Celtics season was the 66th season of the franchise in the National Basketball Association (NBA). The Boston Celtics finished the regular season with a 39–27 won-loss record, which was the 4th best in the East, winning their 21st Atlantic Division title. Their longest winning and losing streaks were 5 games. The leading scorer was Paul Pierce, averaging 19.4 PPG. The leading rebounder was Kevin Garnett (8.2 RPG). Rajon Rondo led the team and the league in assists per-game with 11.7. The regular season was reduced from its usual 82 games to 66 due to the lockout. The Celtics made a relatively deep playoff run, where they defeated the Atlanta Hawks in six games in the First Round, and the Philadelphia 76ers in seven games in the Semifinals, eventually challenging the Miami Heat for the third consecutive season (the team who defeated them in last season's Semifinals), in the Eastern Conference finals. They ultimately lost the series in seven games to the eventual NBA champion.

Following the season, Ray Allen departed via free agency for the eventual champion Miami Heat, effectively ending the Big 3 era in Boston. During his 2 years as a Heat, Allen would help them win a second-straight title the following year over the San Antonio Spurs in seven games, and appear again in the 2014 Finals against the same Spurs team.

It was also the NBA's first time without Shaquille O'Neal since 1991–92, as he retired in May 2011 and played his final season as a Celtic.

• June 23: The 2011 NBA draft took place at Prudential Center in Newark, New Jersey.^[1]
• December 25: The regular season begins with a matchup against the New York Knicks in a rematch from the previous season's playoffs.
• April 18: The Celtics secure the Atlantic Division title in a 102–98 win against the Orlando Magic.^[2]
• April 29: The 2012 NBA Playoffs started.
• May 10: The Celtics advanced to the Eastern Conference Semi-Finals, defeating the Atlanta Hawks.
• May 25: The Celtics advanced to the Eastern Conference finals, defeating the Philadelphia 76ers
• June 9: The Celtics were defeated by the Miami Heat and were eliminated from the NBA Playoffs.

• Paul Pierce was named Eastern Conference Player of the Week twice^[3][4] (January 30 – February 5 and March 26 – April 1).
• Kevin Garnett was named Eastern Conference Player of the Week (April 9 – 15).^[5]
• Paul Pierce and Rajon Rondo participated in the 2012 NBA All-Star Game. It was their 10th and 3rd appearance respectively.
• On March 22 Paul Pierce surpassed Clyde Drexler on the All-time scoring leaders table in a 100–91 win against the Milwaukee Bucks.^[6]

• Jeff Green was not available for the 2011–12 season due to recovery from a heart surgery.^[7]
• Chris Wilcox was out for the season in March with a cardiac irregularity.^[8]
• Jermaine O'Neal was out for the season on March to have surgery on his left wrist.^[9]
• Rajon Rondo earned a two-game suspension for throwing the ball to a referee during a game against the Detroit Pistons on February 19^[10] and was suspended for one game during the playoffs for bumping a referee on game 1 of the first round against the Atlanta Hawks.^[11]

• 2011–12 NBA season
• Uncut Gems, a 2019 film by the Safdie brothers starring Adam Sandler, set around the 2012 conference semi-finals; features Kevin Garnett as himself, with archival footage of Games 5-7.