Rapper iLoveMakonnen went DOWN on Tuesday night -- instead of "goin' up" like he says in his hit song -- when he was attacked on stage by a fan.
iLoveMakonnen was performing his hit 'Tuesday' at SOB's in NYC ... when a large man in a grey hoodie walked up to the front of the stage (:08 in the video), and then grabbed the rapper and started throwing haymakers.
Watch the clip ... after getting in several hard shots, the attacker actually managed to slip away -- but sources inside the club tell TMZ the owners know exactly who the guy is ... and describe him as a "jealous fan." We're told he's been banned for life.
Cops never got involved -- no one called 'em -- and iLoveMakonnen wasn't seriously hurt.