With the Raptors leading 116-115 with four seconds remaining, Celtics guard Isaiah Thomas drove hard to the hoop and had his shot deflected by Raptors guard Louis Williams. Smart caught the ball and put it back up just before time expired for his first career buzzer beater.
Smart finished with 15 points, four rebounds and four assists. Thomas led the Celtics with 25 points, while center Tyler Zeller added 20 points and nine rebounds and guard Evan Turner scored 18 points and recorded 10 assists.
Raptors guard DeMar Derozan led all players with 38 points on 14-for-25 shooting, including 10 for 12 from the free-throw line.
The win moved the Celtics to 35-42 and into the Eastern Conference's eighth spot with the Miami Heat's 99-98 loss in Detroit. The Raptors, who are fighting for the No. 3 seed, dropped to 45-32.