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 .