The talk show host makes the admission during Pitt's surprise appearance on her program "The Ellen DeGeneres Show" on Friday, September 13.
During the interview, Pitt, 55, recalled a party at singer-songwriter Melissa Etheridge's house in the '90s where he met the 61-year-old comedian for the very first time.
"I remember Melissa Etheridge's pool party where I think you were hitting on my girlfriend," the father-of-six told Ellen DeGeneres.
"Was I hitting on your girlfriend?" a surprised DeGeneres asked the "Once Upon a Time in Hollywood" star. "Really?"
Pitt assured DeGeneres she was definitely flirting with his date, but said he "was flattered."
DeGeneres then dropped this bombshell: "I actually sense that I have dated another one of your girlfriends. We'll talk about that later," as Pitt laughed at the revelation.
In a clip posted on the show's Instagram account, the "Fury" actor surprised DeGeneres and her studio audience as he sat among them to catch the taping of the show.
"Have a seat everybody, go ahead and catch your breath, Twitch," she told show DJ Stephen "tWitch" Boss. "There's something I want to talk about - I'm sorry, I'm kind of distracted right now."
At that point, he stood up in the audience, which wildly cheered as he clapped along with them as Pitt greeted the talk show host, 'Hey Ellen,' from the audience.