The 26-year-old star faced off with the talk show host in a game of 'Water War', which left him completely soaked, just minutes before he had to go to a premiere, reported Contactmusic.
"You know what, for the record as well, just so everyone knows, I'm going to a premiere after this and I don't think I've brought more underwear," he said.
As the game ended, Radcliffe had to endure one more soaking as Fallon fired a water gun in his face. Sporting a newly shaven head, the star took it all in his stride, saying 'thanks for having me'.
As Fallon argued with audience members who tried to make him do the dance, Radcliffe was shown giving him a little extra encouragement.