Per Shams Charania of Stadium and The Athletic, Smart has agreed to a four-year extension worth $77.1 million that will keep him with the Celtics through the 2025-26 season.
Smart appeared to confirm the deal on social media:
He also posted "Let's got 4 more..." on Instagram.
Boston got an early start on offseason trade activity by agreeing to send Kemba Walker, the No. 16 overall pick in the 2021 NBA draft and a 2025 second-round pick to the Oklahoma City Thunder for Al Horford, Moses Brown and a second-round pick in 2023.
Swapping Walker's salary ($36 million) for Horford's ($26.5 million) gave the Celtics an additional $9.5 million in cap space to use on roster improvements this summer.
Brown was used in a trade with the Dallas Mavericks to acquire Josh Richardson.
Smart was set to enter the final season of his previous contract that pays him $13.8 million. The 27-year-old set career highs in scoring (13.1 points per game) and assists (5.7) in 48 appearances last season.
Even though Smart is not an efficient scorer, his defensive ability has made him one of the most valuable all-around players in the NBA. The Oklahoma State alum was named to the All-Defensive first team in the 2018-19 and 2019-20 seasons.
Another big part of Smart's success in the NBA is his versatility. He has appeared in 449 games in the past seven seasons, with 228 of those coming in the starting lineup. The Celtics have used him at point guard and shooting guard.
Brad Stevens, who is in his first season as Boston's president of basketball operations, knows very well what Smart is capable of doing and how important he is to the franchise's success.
By committing to the Celtics now, Smart has given the franchise more flexibility to flesh out its roster for the short and long term around a trio of himself, Jayson Tatum and Jaylen Brown.