Boston Celtics giving Tatum record $A471m deal: report

Jayson Tatum is the newest member of the NBA's $300 million club.

The Boston Celtics star has agreed to a five-year, $US314 million ($A471 million) contract extension to remain with the reigning NBA champions, according to a person with knowledge of the situation, who spoke on condition of anonymity because it has not been announced.

They said the new pact will begin with the 2025-26 season and keep the five-time All-Star in Boston through the 2029-30 season.

The Athletic and broadcaster 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 off-season by teammate Jaylen Brown, when he inked a five-year deal that will pay him up to $US304 million.

Tatum averaged 26.9 points, 8.1 rebounds, 4.9 assists and shot 47 per cent from the field in earning All-NBA first-team honours 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 Tatum eclipsed 30 points six times, including 31 in their championship-clinching Game 5 win over the Dallas Mavericks.

He will now look to add to his hardware collection as a member of the US men's basketball team when they try to win a fifth consecutive Olympic gold medal. Tatum previously won gold at the Tokyo Olympics.

News of Tatum's deal came on the same day that fellow Celtics starter Derrick White reportedly agreed to a four-year, $US126 million ($A189 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 came on the heels of the Celtics' ownership group, led by Wyc Grousbeck, announcing plans to sell all its shares by 2028.

Barring any significant roster changes, Boston are expected to have a combined payroll and luxury tax that will exceed $US400 million ($A600 million) when Tatum's contract takes effect.