The new pact will begin with the 2025-26 season and keep the five-time All-Star in Boston through 2029-30 season, according to the person who spoke on the condition of anonymity because it has not been announced.
The Athletic and ESPN were first to report the deal.
When completed Tatum's new, supermax contract will become the largest in NBA history, supplanting the one signed last offseason by teammate Jaylen Brown, when he inked a five-year deal that will pay him up to $304 million.
Tatum averaged 26.9 points, 8.1 rebounds, 4.9 assists and shot 47% from the field in earning All-NBA first-team honors for the third consecutive season. Those numbers grew to 25 points, 9.7 rebounds and 6.3 assists per game during a playoff run in which he eclipsed 30 points six times, including 31 in their championship-clinching Game 5 win over the Dallas Mavericks.
He will look to add to his hardware this summer as a member of the U.S. men's basketball team when it attempts to capture its fifth consecutive gold medal. Tatum previously won a gold medal at the Tokyo Olympics in 2021.
News of Tatum's deal came on the same day that fellow Celtics starter Derrick White reportedly agreed to a four-year, $126 million extension. All five of Boston's starters -- Tatum, Brown, White, Jrue Holiday and Kristaps Porzingis --- were already under contract for next season. With Tatum and White's new agreements they now all could be together through at least the 2025-26 season.
The deal also came on the heels of the Celtics ownership group, led by Wyc Grousbeck, announcing its plans to sell all of its shares by 2028.
Barring any significant roster changes, Boston is expected to have a combined payroll and luxury tax that will exceed $400 million when Tatum's contract takes effect.