The `Twilight Saga` hunk was putting money into a homeless busker`s tin in Los Angeles when he noticed how battered his instrument was and so headed into a nearby music shop to splash out on a gift for the man. He then emerged with an electric guitar for himself and a new red acoustic for the busker.
British builder Luke Jones witnessed the actor`s generosity, and told The Sun newspaper: "We were gobsmacked when he handed the acoustic guitar to the tramp.
"Robert quickly went back to his car and drove off.
"The busker was totally shell-shocked. He had no idea who Robert was - I think he thought the guitar was stolen.
"He was looking left and right, half expecting the police to arrest him. We asked him to play but he couldn`t get his thoughts together."
A standard new acoustic guitar costs around £50.
Robert - who plays vampire Edward Cullen in the vampire series - is a keen musician himself and has recorded two tracks, `Never Think` and `Let Me Sign` for the soundtrack to the first `Twilight` film.
The 24-year-old star has previously said he sees music as his "back-up plan" if his film career fails, and that he believes he will be famous when he`s dead.
He said: "I am sure I`ll be one of those artists that people don`t pay any attention to while I`m alive, and when I am dead people will go yeah, man, that was a work of art."