The US actress and singer couldn't help "blushing" when pressed on the whether the pair are an item, by chat show host David Letterman.
But she told him: "He's been in my life for so long. It's just nice to have someone that understands what I'm going through."
Letterman showed her a picture of the couple taken last month at the Vanity Fair Oscar Party, to which the former Disney star replied: "Oh, that's cute."
Selena, 18, said they'd known each for "almost two years", and said their managers were behind their first meeting.She told the Late Show With David Letterman, recorded in New York: "His manager called my manager, who was my mom."
Letterman responded: "I heard he was your paper boy" to which Selena, wearing a black lace bustier top and black and white print skirt, admitted: "I'm blushing."
Photos of Selena and 17-year-old Justin at the post-Oscars party last month, showed them snuggling together and looking like a couple, and were taken as confirmation that the pair were dating.
Teen star Justin has said: "I don't think it's anyone's business what my private life is. Sometimes you have to keep some stuff to yourself."
