Jonathan Cheban's Bodyguard Punches Knife-Wielding Guy ... Over Sushi [VIDEO]

TMZ has obtained video of reality star Jonathan Cheban's bodyguard socking an aggressive stranger in the face.

Cheban ... a fixture on "Keeping Up With The Kardashians" ... was grabbing a late dinner with his bodyguard and 3 friends at Sushi Couture in Brooklyn Sunday night when a random stranger started heckling Jonathan.

We're told Jonathan Instagrammed he was at the restaurant ... then a preppy-looking dude with a knife hanging off his belt approached his table, but was blocked by bodyguard "Sal the Contractor." The guy even followed Jonathan to the bathroom.

Sources in the restaurant tell us the instigator waited until the end of the night, to lunge at Jonathan as his entourage got up to leave. That's when Sal slammed a punch into the dude's face, as our video shows.

We're told the guy backed down and left after the punch. Smart.