Rapper Rich Homie Quan -- Boxing Match Brawl IN THE CROWD!

Rapper Rich Homie Quan got into a brawl at a boxing match Friday with a guy he claims was trying to steal his chain -- and the fight video is pretty intense.

Quan was at the Adrien Broner fight in Cincinnati when the guy allegedly tried to steal the bling with force ... the rapper started throwing punches to thwart the attack -- which he did.

The video doesn't show the attempted snatch ... just lots of chaos.