Cardi B Twerks in Crowd, Smacks Own Butt at REVOLVE Festival

Cardi B gave fans at a REVOLVE Festival a private concert on her "Bongos" ... smacking her booty like a drum while twerking at the exclusive event.

The rapper took the stage in Thermal, California ... forgoing Coachella for the invite-only hip hop-focused fest -- and, at one point during the show, she bent forward at the waist and went into overdrive.

Check out the clip ... Cardi's standing above the crowd -- twerking and smacking her booty while fans film and a few even reach out to smack her too, with which it seems she's surprisingly cool.

It's not just a couple seconds of dancing BTW ... Cardi gets low -- almost certainly getting fans to yell out "I Like It."

Cardi was just one of many big stars who performed at the ... with the one-day festival featuring hip hop heavyweights like Lil Wayne, Tyga and LiAngelo Ball also taking the stage.

The festival isn't something just anyone gets tickets too ... with admission only going out to big-time influencers and other high-profile stars.