After helping the Boston Celtics win the 18th NBA championship in franchise history last season, Jayson Tatum is going to sign a long-term extension to remain with the club.

Per Chris Haynes of TNT Sports, the 26-year-old has agreed to a five-year, $315 million contract extension with the Celtics.

The deal is the largest in NBA history.

Tatum was previously playing on a five-year, $195 million max extension signed in November 2020. He received that deal after making his first All-Star team and leading Boston to the Eastern Conference Finals during the 2019-20 season.

By virtue of being named to the All-NBA first team during the 2022-23 season, he became eligible to sign a supermax in the summer of 2024.

The Celtics' cap situation was made more complicated with the 2022-23 All-NBA team announcements. In addition to Tatum making first team, Jaylen Brown's inclusion on the second team made him eligible for a supermax extension.

Brown wound up signing his five-year, $286.2 million extension with the Celtics on July 26.

Boston also added Kristaps Porziņģis in a three-team trade with the Memphis Grizzlies and Washington Wizards that cost the team Marcus Smart, Danilo Gallinari and Mike Muscala.

Porziņģis opted in to his $36 million salary for 2023-24 to facilitate the deal. The 27-year-old is signed a two-year, $60 million extension through the 2025-26 season over the summer.

Jrue Holiday, who was acquired from the Portland Trail Blazers shortly before the start of the 2023-24 season, got a four-year, $135 million extension from Boston in April.

Derrick White and the C's agreed to a four-year, $125.9 million extension on Monday that kicks in for the 2025-26 season, per ESPN's Adrian Wojnarowski.

Now that the Celtics plan to move forward with Tatum and Brown on supermax deals, that duo will account for approximately $107 million in 2025-26, $116 million in 2026-27, $124 million in 2027-28 and $132 million in 2028-29.

Boston is one of three teams projected to be over the second tax apron in 2024-25. This was going to happen regardless of Tatum's new deal because he was already under contract, but the apron could impact other things the front office might look to do this offseason.

Even with some potential financial and roster limitations because of the second apron rules, the Celtics are still in prime position to once again be the top team in the Eastern Conference in 2024-25.

Boston's front office has done a great job over the years of being proactive in getting players signed to deals that give them a lot of money and are also palatable for the organization with the cap.

Regardless of the long-term cap implications for the Celtics, extending Tatum had to be an easy decision. He has finished in the top 10 in NBA MVP voting in each of the past three seasons.

The Duke product averaged career highs in scoring average (30.1 points per game), assists (4.6) and rebounds (8.8) during the 2022-23 campaign. He's the face of the franchise and still in the early stages of his prime.

Tatum followed that up by averaging 26.9 points on 47.1 percent shooting, 8.1 rebounds and a career-high 4.9 assists per game in 74 starts last season. He put up 25.0 points, 9.7 rebounds and 6.3 assists per contest in 19 starts during the Celtics' postseason run to the NBA title.

The Celtics have made the playoffs every year of Tatum's career, including five trips to the Eastern Conference Finals and two appearances in the NBA Finals in the past three years.