Vin Diesel -- 1-Man Dance Party ... to Katy Perry Jam

Vin Diesel felt the urge to dance ... by himself ... to a Katy Perry song ... and thankfully, he recorded the entire session on video to share with the world.

Diesel took a break in the middle of rockin' out to explain why he felt like dancing -- "Universal just called me and told me "Riddick" is number one on the DVD charts."

Vin says he's especially stoked because it was such a challenge to get the movie made in the first place -- so to celebrate, he cranked some KP (and later Beyonce) and let himself go.