The 26-year-old star, who is sporting a completely buzzed hairstyle for his latest movie role as a government agent in the Daniel Ragussis-directed flick Imperium, knows what to expect when it comes to Fallon's celebrity talk show games.
And totally up for the challenge, the handsome actor was eager to get going with his game of Water War.
After some initial switching of cards, Radcliffe won the first round and laughed with glee as he chucked a glass of water over the show host's head.
In round two, it was the Harry Potter star's turn to get soaked, but then the British actor was on a roll, soaking Fallon time after time, thanks to his good cards.
Even when the duo pulled the same card, and Fallon declared war, Radcliffe was victorious, getting to pour an entire jug of water over his opponent's head.
Eventually, of course, Fallon got his own back, pouring a glass of water right into his guest's lap!
"For the record I'm going to a premiere later," Radcliffe told the audience, "and I don't think I bought any other underwear!"
Watch the video to see what happened on the final round where the stakes were raised and the loser got water cannoned!
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