Katy Perry Does Will Smith Karaoke Rap

It all went down at a karaoke joint on Catalina Island called El Galleon ... Katy went up on stage under the name "Katherine" (smooth), along with another female friend of hers, to get jiggy with Smith's 1998 tune.

The footage is pretty great ... the pop superstar gets most of the words right and even manages to put her own twist on it by saying things in a really high voice -- which was interesting.