The talk show host, 40, challenged Gyllenhaal, 33, to one of Fallon's famed water wars.
The pair took a seat in facing chairs with a small table between them. Surrounded by various glasses and pitchers full of water, they prepared for an intense round of the card game war. Because it wouldn't be The Tonight Show without a twist, Fallon upped the ante on lost hands — hence the need for the water.
"We're playing the card game war but if you lose the hand, you get a pint of water in the face," the funnyman said. "We each have five glasses of water; the first one to throw all five glasses on their opponent wins, and then gets to water cannon the loser."
Fighting back giggles, Gyllenhaal and Fallon kicked off the game, flipping cards and getting soaked as they engaged in a little water war.
The game heated up when Fallon tried to play mind games with Gyllenhaal, offering to switch decks with him, and quickly defeating him in an all-out three-card war.
Watch both stars get soaked in water in the video above!