The 'Bodies' singer appeared on the site, which randomly sets up one-to-one webchats, last week.
Adam Houabi, 19, was given a private performance of a brand new song, which Williams explained was a new song titled 'Losers', and will be released in October.
"One minute I was talking to a bloke from Algeria... then amazingly Robbie appeared," Houabi told The Sun.
"I didn't believe it was really him so he pulled his shirt down to reveal his chest tattoos and showed me five cameramen round him in his Hollywood studio. Then Robbie told me he was recording a new song and started singing.
"It was good. I liked it. I've chatted to a few strange people on Chatroulette before, but never a superstar!"
Williams is heard in the filmed chat telling Houabi: "You've been a beautiful person, the song is called 'Losers'. It should be out in October some time.
"God bless you, take care to you in Essex, from me in Los Angeles. You're the man, see you dude!"
However, a spokesperson for Williams has stated that the track will not be his next single.
Williams is expected to release his ninth studio album in the autumn.