Rihanna Grinds Drake On Stage -- How's My Ass Feel? [VIDEO]

Rihanna and Drake got it on last night in Paris ... in front of 17,000 people ... grinding on stage to their duet "Take Care" -- and it's all on video.

As we reported, Drake and Rihanna have been inseparable in Paris since Drake flew into town this weekend for his tour, getting dinner, hanging out at nightclubs ... even crashing together at the same hotel.

And the chemistry on stage last night is undeniable -- take a close look at Rihanna's face.

There's no way she's acting either ... because she sucks at that.