NBA executive vice president of basketball operations Kiki VanDeWeghe said in a statement the amount of Smart's fine reflects his "multiple prior violations of acceptable on-court decorum."
The incident occurred after the final buzzer of the Celtics' 129-120 overtime loss to the Brooklyn Nets on Tuesday night. Boston led the Nets by three points with 0.2 seconds left when officials called a foul on Smart, who was guarding Caris LeVert on a 3-point shot.
LeVert made all three of the free throws to tie the game and send it into overtime. Smart later fouled out in the extra quarter as the Nets went on to win the contest.
After the defeat, Smart approached two referees and had a heated conversation with them. He began to yell and point at the officials before being pulled off the court.
Smart previously was fined $15,000 for criticizing officials earlier this season. He was disciplined three times in the 2018-19 campaign for his actions, including a couple of altercations with opposing players that cost him more than $100,000 in fines.
Smart has averaged career highs in points per game (13) and assists per game (4.9) in 50 contests (36 starts) this season, his sixth with the Celtics.