Boston Celtics center Al Horford has agreed on a two-year, $20 million contract extension that ties him to the franchise through his 39th birthday in 2025, his agent Jason Glushon of Glushon Sports Management told ESPN on Thursday morning.

Horford, a five-time All-Star, is completing the final season of a four-year, $109 million contract. Horford will earn $26.5 million this season, with the new deal beginning in 2023-24.

The extension eliminates the possibility of a repeat of 2019, when Horford left the Celtics for a more lucrative free agency offer. The possible free agent centers available next summer include Indiana's Myles Turner, Chicago's Nikola Vucevic, Milwaukee's Brook Lopez, Dallas' Christian Wood and San Antonio's Jakob Poeltl.

Horford has had a career revival in his second run with the Celtics, emerging as an integral part of a championship contender. At 36, he has remained a productive player, and he continues to hold a significant amount of respect inside the locker room and in the Boston community.

Horford was most recently an All-Star and a second-team All-Defensive team selection in 2018.

Horford's appearance in the NBA Finals a season ago ended the league's longest playoff streak -- 141 postseason games -- without winning a conference championship, according to ESPN Stats & Information research.

Horford is averaging 10.9 points and 6.3 rebounds in 31 minutes a game this season, anchoring the Celtics' front line in the absence of injured starting center Robert Williams III. Horford has shot 55.5% from the floor, and 48.8% on 3-pointers -- the third-highest percentage among centers, according to ESPN Stats & Info data.

Horford's Game 1 performance against Golden State made him only the fourth player in the shot-clock era to score 25 points on 75% shooting in a Finals debut, a list that included Kareem Abdul-Jabbar.

No player contested more shots in the 2022 NBA playoffs -- 437 -- than Horford, according to Second Spectrum tracking. He's one of five players to contest at least 400 shots in a single postseason since player tracking began a decade ago.

Horford left Boston for Philadelphia on a four-year, $109 million free agent deal in 2019, then landed in Oklahoma City in a trade in 2020 before the Celtics reacquired him in 2021.

Horford, the No. 3 pick in the 2007 NBA draft, out of Florida, played his first nine NBA seasons with the Atlanta Hawks before leaving in free agency for Boston in 2016.

Horford played three years with the Celtics before the stops with the 76ers and Thunder and his 2021 return to Boston.