The hunky actor was on the show to talk about his new movie Nightcrawler, but when talk about his character inadvertently turned to fish (and in particular salmon) the cheeky actor couldn't help going off on a bizarre tangent.
"Did I hear salmon?" yelled Gyllenhaal excitedly. "That's my favorite new thing to do," he said. "I need someone to do it to."
"Do it to me," offered Letterman, indulging his guest.
The Brokeback Mountain star jumped up and instructed Letterman to stand alone and do a monologue. Mid-way through his spiel, the bearded actor ran up to the talk show host and slapped his hand, side to side, on his inner thighs while yelling the word, “Salmon!”
After a rapturous reception from the audience, Letterman returned to his seat. "That's the nicest thing anyone's ever done for me," he told a happy-looking Gyllenhaal. "For a moment I thought I was standing in the middle of a North Atlantic stream and there were actual salmon spawning."
"It's a visceral experience," replied the actor, nodding.
"It's nice, I enjoyed it and it's continuing to tingle," said Letterman, cracking up.
Watch the whole bizarre episode on the video above!