The NBA is making Marcus Smartpay 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.
The NBA just handed down Smart's punishment for the freak-out -- fining Smart $35k for "confronting and verbally abusing the game officials." The NBA says Smart's history of going off on refs factored into the $35k figure.
As for the game itself, Tuesday was a disaster across the board for Boston -- Brooklyn's LeVert scored a career-high 51 while the Celtics gave up a whopping 51 points in the 4th quarter ALONE to seal the loss.
FYI -- Smart famously signed a $52 million deal back in 2018 (remember his epic rap song?) ... so really, just a drop in the bucket for the dude.