EXCLUSIVE

Finally, Michael Phelps broke something that wasn't a world record -- HIS ASS -- taking a nasty spill while crowd surfing in a Vegas nightclub this week ... and TMZ has the footage!

Phelps -- who's STILL in party mode after the London Games -- hit up XS nightclub Monday night, and for some reason thought it would be a good idea to crowd surf inside an inflatable raft.

It was not ... because the 18-time gold medalist barely made it a few feet before toppling backwards onto his ass.

Thankfully, we're told he was okay -- but the video's still awesome. Check it out.

Thanks to our friends at papUrazzi.com for the clip.