During Wednesday's interview, Letterman held up a Feb. 27 photo of Gomez, 18, and Bieber, 17, together at the Vanity Fair Oscar party and asked: "He was your date? You're dating -- are you dating? You're not dating?"
Gomez replied: "It was like a little prom night, wasn't it? Looked like it."
"You guys look very happy there," Letterman remarked. "How long have you known him?"
"Uh, almost, like, two years now," Gomez said.
Letterman then inquired how they met, to which Gomez said Bieber's manager called her mother, who's also her manager.
"Are you dating? You're not dating?" Letterman followed up.
"He's been in my life for so long," Gomez said, "and it's just nice to have someone that understands what you're going through."
Letterman went on to wish Gomez and Bieber luck, to which Gomez said, "Thank you."