League sources told ESPN, The Athletic and the Boston Globe on Monday that Smart is signing a four-year, $77.1 million deal with the Celtics. The extension, which begins in the 2022-23 campaign, is the maximum allowable off Smart's $14.3 million salary for the upcoming season.
The agreement doesn't include a player option, according to ESPN. The Athletic reported that Smart's deal includes a trade kicker.
Smart later confirmed the extension with a post on social media, writing: "Let's go 4 more."
Smart, who has played his entire seven-year NBA career in Boston, has developed into one of the team's most important pieces. The fiery guard has been voted to the NBA's All-Defensive First Team twice.
The 27-year-old Smart averaged career highs in points (13.1), assists (5.7), blocks (0.5) and minutes (32.9) for the Celtics this past season.
Boston selected Smart with the No. 6 overall pick in the 2014 draft out of Oklahoma State. He has averaged 10.2 points, 4.2 assists, 3.6 rebounds and 1.6 steals over 449 career games (228 starts).