The Boston Celtics continued to celebrate their 18th NBA championship by rewarding star forward Jayson Tatum with a record-breaking contract extension.

The Celtics and Tatum agreed to a five-year supermax extension worth nearly $314million, multiple media outlets reported Monday.

The contract is the largest in NBA history, with Tatum qualifying for a supermax deal by being voted to the All-NBA First Team in 2022-23.

Tatum’s deal is the latest in the Celtics’ effort to keep their championship core in place into the foreseeable future, despite a massive luxury tax bill.

Multiple media outlets reported that Boston and Derrick White agreed to a four-year, $126million extension earlier Monday.

Finals MVP Jaylen Brown signed a five-year, $286million extension last summer. Kristaps Porziņģis and Jrue Holiday are both signed through at least the 2025-26 season.

Even before agreeing to Tatum’s extension, the Celtics were projected to be over the second apron luxury tax, which restricts roster transactions by the league’s highest-spending teams.

Tatum, 26, is already a five-time All-Star and has been voted to the All-NBA First Team in each of the last three seasons.

Tatum averaged 26.9 points, 8.1 rebounds and a career-high 4.9 assists last season while leading the Celtics to a 64-18 record.

He has helped Boston to two NBA Finals appearances and has already played in 113 play-off games.

In 513 career regular-season games, Tatum averaged 23.1 points, 7.2 rebounds and 3.5 assists.