The host, who's famous for playing practical jokes on her guests, found herself on the receiving end of a prank when Cooper, 37, stopped by to promote his acclaimed film Silver Linings Playbook and gave the comedian a taste of her own funny medicine.
"Hey, do you remember when you asked me last time if I could take my shirt off?" the Golden Globe nominee asked, eliciting excited screams from the audience. "I remember thinking, 'Well, I don't know if I should reveal this much about myself.' I didn't do it, because…I'll show you why. I'm gonna do it. I feel free with you guys."
Cooper then unbuttoned his red shirt to reveal one of his famously defined pecs -- and a surprise third nipple.
"You have three nipples?!" DeGeneres exclaimed.
"I actually have -- this is so sick -- I have a fourth nipple," he teased, rolling up his right sleeve to reveal a "nipple" on his forearm. Then, hiking up his pant leg, he said, "I have a fifth one, too."
"What is this, show-and-tell?" DeGeneres joked of the gag. "You're just wasting time."
Earlier in their chat, the sexy star gushed about starring opposite Jennifer Lawrence in Silver Linings Playbook, for which both are nominated for Golden Globes.
"For me, I hit the jackpot getting this role," he said. "David O. Russell's one of the best directors we have. So to be able to be in one of his movies, and to be able to play a character like that…and then I got to work with Jennifer Lawrence, who is unbelievable. Not only is she a great actor but she's an incredible person."