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.