The Harry Potter star admits he struggles to keep up with current hot pop stars, but had heard Bieber's hit Baby on the radio.
However, the out-of-touch Brit believed the 16-year-old's high-pitched vocals belonged to a female.
He tells MTV.com, "I only heard Justin Bieber for the first time two weeks ago. I genuinely thought it was a woman singing. I'd never heard it before.
"I'm so out of the loop when it comes to pop culture that I kind of don't even try anymore."
But there is one superstar the Brit can't help but notice - he's a huge fan of Lady GaGa.
He adds, "I know who Lady GaGa is. I think she's kind of amazing. She's really got a voice. My God. Not my kind of music, particularly, but she can really sing."