Lady Gaga -- Crotch-Thrusting Into Birthday Cake [Video]

Lady Gaga stripped down to her thong and banged a piece of birthday cake ... and thankfully, Terry Richardson captured the entire thing on video tape.

Richardson -- the god-like genius who also directed the video of Kate Upton doing the Cat Daddy in a bikini -- released the clip on Gaga's official YouTube page.

And if you're concerned about Gaga's personal hygiene after her romp with the dessert ... don't worry, she takes a bath at the end of the clip.