The Boston Celtics and All-NBA forward Jayson Tatum are reportedly finalising a five-year extension worth $US314 million ($A471 million), which would be the largest deal in NBA history.

The deal, reported by broadcaster ESPN, comes after Tatum helped guide the Celtics to the NBA title, the 18th in franchise history.

Tatum, 26, averaged 26.9 points, 8.1 rebounds and 4.9 assists per game in 74 regular-season starts, before adding 25.0 points, 9.7 boards and 6.3 assists a game in Boston’s 19-game romp through the playoffs.

Tatum was an All-NBA first-team selection for the third straight year and has been named to five All-Star Games.

He has spent his entire seven-year NBA career in Boston and has career averages of 23.1 points, 7.2 rebounds and 3.5 assists per contest over 513 games.

The Celtics also locked up starting guard Derrick White to a $US126 million ($A189 million) contract extension.