"It's an honor to have this opportunity to once again call myself a Boston Celtic," Pierce said in the team's press release. "The organization and city took me in and made me one of their own, and I couldn't imagine ending my career any other way. I'm a Celtic for life."
Last October, Pierce said the 2016-17 season would be his last and that he eventually planned to join Boston to bring his career full circle.
The Celtics selected Pierce with the 10th overall pick in the 1998 NBA draft. He spent 15 seasons with the team, appearing in the third-most games (1,102) in franchise history. His 24,021 points are also second all time among Celtics players behind John Havlicek (26,395 points).
Pierce helped Boston reach the playoffs on 10 occasions and was a key piece of their title-winning team in 2007-08.
The 39-year-old has wasted little time transitioning to a second career with his time in the NBA over. He began working for ESPN as part of the network's NBA Finals coverage, and Sports Illustrated's Richard Deitsch reported in June that ESPN was interested in hiring Pierce full-time beginning with the 2017-18 season.