NBA executive vice president of basketball operations Kiki VanDeWeghe said the amount of the fine reflects Smart's "multiple prior violations of acceptable on-court decorum."
Smart had some words for officials after the final buzzer of Boston's 129-120 overtime loss to the Nets and had to be restrained before being led into the tunnel.
The Celtics led by three points with 0.2 seconds remaining when a foul was called on Smart, who was defending the Nets' Caris LeVert on a 3-point attempt. LeVert made all three free throws to tie the game and send it to overtime. LeVert finished with a career-high 51 points.
The Celtics blew a 21-point lead in the second half, and the Nets scored 51 points in the fourth quarter to force the overtime and eventually secure the victory. Smart fouled out in the overtime period.
Smart had been fined $15,000 for criticizing officials earlier this season and was penalized three times in 2018-19 for his actions, including a couple of altercations.
The Associated Press contributed to this report.