The New York-born rapper shocked the packed crowd at the Kia Forum in Inglewood when she strutted out onstage without warning for a sexy two-minute-plus performance with Madonna.

Cardi's surprise appearance occurred during the "Vogue" segment in which Madonna usually brings out other celebs onstage to rate a raunchy dance-off as part of her "Celebration" tour.

Check out this video obtained by TMZ … Cardi and Madonna were seated in silver chairs next to each other holding signs emblazoned with the number "10" and the word "Chop."

The superstars rated the dancers pirouetting around them and worshipping their bodies, giving them all, of course, tens. Cardi also jumped into the contest, twerking her butt off. And you know she scored a ten!

At one point, Madonna held up the "Chop" sign, which in ballroom competitions means you're not advancing to the next round. The audience probably didn't catch that, but who cares … Madonna, Cardi and the rest were having a blast!

Meanwhile, Madonna got a little raunchy herself with Cardi, caressing her thighs throughout the segment. But that’s just expected with Madonna and Cardi didn’t seem to mind one bit.

As the song wrapped, Cardi was escorted by the dancers off the stage ... and Madonna just kept the show going.