The 46-year-old surf legend pulled an UNBELIEVABLE move during a competition in Hawaii -- falling off his board and getting back on in the same ride -- and it was all captured on video.

Kelly was riding against Filipe Toledo at the Billabong Pipe Masters competition in Oahu over the weekend ... when he pulled a move that's already being dubbed, "Houdini Tube Ride."

Long story short ... Slater entered the wave and got knocked off -- but somehow managed to grab his board during his fall and placed it back under his feet during the ride.

He miraculously emerged standing on the board -- and looked at the crowd like he could barely believe it himself!!!

Dude is amazing!!!!

Slater later commented, "Well, that was interesting. I thought I just fell straight onto my belly and bodyboarded out. I didn't realize I had to grab for it!"