The British actor - who is best known for portraying vampire Edward Cullen in `The Twilight Saga` - was "obsessed" with Eminem as a teenager and made recordings of himself performing his own composition.
He said: "You want a secret? OK, how about this - I`ve got a whole collection of tapes of me rapping from when I was 13! I wanted to be a rapper so there are hundreds and hundreds of tapes. They`re very amusing!
"I was obsessed with Eminem when I was younger. When he first came out I was about 12 and fanatical about his `Slim Shady` CD. I think he`s a genius."
The 24-year-old star even had his own "crew" but admits his lyrics were less-than-authentic.
He explained to Top of the Pops magazine: "Most of my rhymes were stolen off other people anyway. They were all like, `I was raised on these streets.`, which was a complete farce because I was actually raised in quite a nice area of London called Barnes. Not very ghetto at all!
"I was obsessed with it all, though. I even had my own little crew - we called ourselves Big Tub and the Tappy Cats."
Robert has previously recorded tracks for the `Twilight` soundtracks but insists he has no plans to move into music full time.
He said: "I`m a bit of an attention seeker - I love being on stage. But the thought of performing music on stage terrifies me. I`m not scared of acting though."
However, if the chance came to perform with rapper Eminem, Robert would jump at the chance.
He said: "I`d love to rap with Eminem! That would be really amazing."