The eccentric pop singer shocked the 30-year-old singer-and-actor when they filmed a 'Saturday Night Live' sketch together after she admitted she was a huge fan of his former group *NSYNC when she was a teenager.
Justin said: "She told me when she was 14, she sat outside a radio station with *NSYNC painted on her cheek.
"She said, 'I hope that doesn't make you feel awkward.' I said, 'Hell no, that makes me feel old.' "
While Gaga was a big fan of Justin growing up, the 'Friends with Benefits' star admits his upbringing in Memphis, Tennessee, didn't allow him to learn much about modern music at the time.
He told Britain's Elle magazine: "Where I'm from is very rural – you don't get pop culture as much. You get that in LA, New York and Miami, on the edges of the country.
"People in the UK think that is America, but most of America is Memphis. More poor people live there than rich. Fashion or culture barely breaks through."