Nothing like a late night water fight!

Twilight star Robert Pattinson and Jimmy Fallon played a little game called Water Wars during RPatz late night appearance ... and seems as though both men lost.

Fallon explained the rules to Pattinson saying: "It's just like the card game War, but with water, so If you lose a hand you get splashed in the face."

After Rob lost the first round, the host told the sexy actor, "I don't want to do this to you, I'm so sorry. Don't be mad at me."

Pattinson responded with the following: "I need to wake up. Hit me!"

Watch the video for the H20 action!