The 2003–04 Boston Celtics season was the 58th season for the Boston Celtics in the National Basketball Association. During the offseason, the Celtics acquired Raef LaFrentz, a teammate of Paul Pierce from the University of Kansas, from the Dallas Mavericks. The Celtics roster move continued in December when Tony Battie and Eric Williams were both traded to the Cleveland Cavaliers for Ricky Davis. In February, Mike James was traded to the Detroit Pistons for Lindsey Hunter and Chucky Atkins in a three-team trade involving the Atlanta Hawks. However, Hunter was waived after the trade and later re-signed with the Pistons. The team also released Vin Baker, who signed as a free agent with the New York Knicks.
Celtics head coach Jim O'Brien resigned after a 22–24 start to the season. He was replaced by John Carroll for the remainder of the season. However, the Celtics would lose 12 of their first 13 games under Carroll. When General Manager Danny Ainge stated that the Celtics would be better off missing the playoffs, the statement seemed to motivate the team as they posted a 9–5 record in March. The Celtics, despite finishing fourth in the Atlantic Division with a low 36–46 record, qualified for the playoffs as the number 8 seed, making them the first NBA team to reach the playoffs with a below .500 record since the 1996–97 Los Angeles Clippers, who were swept by the Utah Jazz that postseason. They were swept in four games by the Indiana Pacers in the opening round. Pierce was selected for the 2004 NBA All-Star Game. Following the season, Carroll was fired as coach and replaced by Doc Rivers, who also fired by the Orlando Magic after 1–10 start.
Neither of the Celtics' home playoff games against Indiana sold out, an increasingly rare occurrence among any NBA playoff teams by the 2000s. The team only drew over 17,300 to Game 3 and about 16,400 to Game 4, thousands of tickets below the FleetCenter's capacity of 18,624. (In fact, neither did Indiana, who only drew as high as 17,000 to Game 2.)