Singer-actor Timberlake, 30, was surprised to hear that the 25-year-old You and I hitmaker used to be a big fan of his boyband NSYNC when she was a teenager.
Justin told British Elle magazine: '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.''
But despite Gaga, real name Stefani Germanotta, being a big fan of his music growing up, the Social Network star says his upbringing in Memphis, Tennessee, didn't allow him to learn much about music at the time.
He added: '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.'