The 2008–09 Boston Celtics season was the 63rd season of the Boston Celtics in the National Basketball Association (NBA), starting out as the defending NBA champions. The Celtics started the season 27–2, which surpassed a mark set by the 1966–67 Philadelphia 76ers for the best two-loss start in NBA history. This run also included a 19-game winning streak, which improved a Celtics franchise record set in 1981–82. However, the Celtics lost seven out of the following nine games, and eventually finished with a 62–20 record.

They failed to defend their title as they lost Kevin Garnett to a season-ending injury. Without their All-Star, the Celtics were knocked out of the playoffs, losing to the Orlando Magic in the Conference Semifinals. The Magic would go on the NBA Finals, only to lose to the Los Angeles Lakers in five games.

On June 26, the Celtics selected shooting guard J. R. Giddens with the 30th and center Semih Erden with the 60th overall picks in the 2008 NBA draft, and traded cash considerations in exchange for small forward Bill Walker, who had been selected by the Washington Wizards with the 47th overall pick. Giddens had worked out with the Celtics before the draft for three days, and received praises from head coach Doc Rivers, who thought he could fight for minutes on the team right away. He was already familiar with future teammates Leon Powe and Kendrick Perkins, who were his teammates at the 2003 McDonald's High School All-American Game. Walker was considered a lottery pick, but suffered the third knee injury in his career during a workout at the Golden State Warriors facility on June 15. However, he remained in the draft despite the injury, and was expected to have minor surgery in July that would keep him out for three to four weeks. Erden was not expected to join the roster this season, and was seen by general manager Danny Ainge as their first-round pick in 2009.

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