Jayson Tatum's summer just got even better -- the Boston Celtics champion just secured a 5-year, $315 MILLION extension ... making it the biggest contract in NBA history.
News of the negotiations broke minutes ago ... with the 26-year-old agreeing to a supermax deal to stay put in Beantown -- quite possibly the biggest no-brainer of the offseason.
Tatum is fresh off winning his first title with the Cs after eliminating the Dallas Mavericks in Game 5 of the NBA Finals last month ... and while he didn't win MVP honors, he was crucial to Boston's success this postseason -- averaging 25 points, 9.7 rebounds and 6.3 assists a game.
The huge payday will start in the 2025-26 season ... with Tatum slated to make $54 million. It will increase roughly $4-5 milli each year, which means he'll rake in a whopping $71 million in 2029-30.
Tatum has emerged as one of the best players in the league after going No. 3 overall to the Celtics in 2017 ... and finished sixth in regular season MVP voting this past season.
Congrats, Tatum ... we take it dinner with the teammates will be on him for a bit.