The rapper passed away Monday -- this according to the group's official social media accounts ... which noted he "went to the upper room." 2LC's manager confirmed his death to us, but didn't have any further details on the cause or manner.

Sources with direct knowledge tell TMZ however that the death appears to be natural, and there isn't foul play suspected.

While Marquis -- who was born as Mark Ross -- wasn't a founding member of the iconic rap group, he certainly helped cement it as the juggernaut act they became in the '80s and '90s of hip hop ... joining the likes of DJ Mr. Mixx, Fresh Kid Ice and Luke Skyywalker in the '86.

By the time Marquis joined 2LC, they were already making waves -- having released "Throw the D" and other raunchy songs that were putting them on the map ... and he only bolstered that with witty tongue-in-cheek lyrics that were full of double entendres.

He ended up appearing on their album 'The 2 Live Crew Is What We Are' ... and went on to be featured as a rapper on other albums of theirs as well, including "Move Somethin'," "As Nasty As They Wanna Be," "Banned in the "U.S.A." and "As Nasty as They Wanna Be Part II."

By the early '90s, most of the Crew was broken up -- but Marquis went on to find more success in the music biz on his own ... forming the group 2 Nasty, and he also went on to be featured on a ton of records as well ... including for MCs like Ice-T and others.

He eventually reunited with some of the 2 Live Crew guys to put out more music, and also released some solo stuff as well.

Marquis was 58.