Justin Bieber -- SMOKES Rapper Action Bronson at Ping Pong

Justin Bieber PRETENDED to be just another guy who tried his hand at ping pong, but it turns out he's world class, much to the dismay of Action Bronson.

Bieber was at SPiN in NYC ... a ping pong social club owned by Susan Sarandon. Bieber and Action were there for a charity NBA All-Star weekend event hosted by Tammy Brook, Rob Stone, Steve Rifkind and Fader Magazine ... DJ Khaled supplied the music.

Action challenged Bieber, who reluctantly said, "ok," but then he kicked Bronson's ass -- 21-0!

What Bieber didn't tell anyone ... we found out he has a rider in his concert contracts requiring a ping pong table, which he plays incessantly.