The NBA is making Marcus Smart pay for blowing up on a ref Tuesday night -- hitting the Boston Celtics star with a big 5-figure fine for "verbally abusing" an official.
The 25-year-old lost his cool after getting called for his 6th foul while contesting a Caris LeVert 3-pointer with 0.2 seconds left during Boston's loss to the Nets ... getting so heated, he had to be escorted off the court.
Smart -- who's no stranger to beefin' with officials -- gave the refs an earful after fouling out, pointing and yelling his frustrations over the call.