Justin Bieber Throws Down in Huge Fist Fight!

Justin Bieber got into an all-out street brawl with a guy much bigger than him Wednesday night in Cleveland.

TMZ obtained this video of Bieber, shot shortly after 11 PM, outside his hotel in downtown Cleveland -- he was face-to-face with the guy, who threw a light blow at Justin's head. That's when Justin uncorked a blow right to his face ... and then it's on!

Several people were standing nearby and tried to jump in, but the fight ends up spilling to the ground.

It's unclear what started the scrap. Bieber was in town for the game 3 of the NBA Finals, and was wearing a Cleveland Cavaliers shirt ... so it could be post-game trash talk. Also, witnesses say Bieber was getting peppered with requests for photos, and we know how he feels about that.

Overall ... we'd say Justin held his own, though he ended up on the bottom of the pile.