Katy Perry -- Groped By Girl Onstage, 'She's Rolling' (VIDEO)

Katy Perry got some girl on girl action from a "rolling" fan she brought onstage at the Rock In Rio festival.

Things started off sweet with the fan showering Katy with a few kisses, and then she decided to cop a feel of Katy's boobs -- which is when KP reaches the conclusion, "She's rolling!"

The girl's name remains a mystery ... even after she slurred it multiple times onstage. Watch the video, you'll see what we mean, but gotta hand it to Katy -- she handled it like a champ.

The whole exchange is hysterical, right up to the fan's grab-ass exit.

Bad KatyCat.