The singer, who released an album called Take The Crown last year, said he had the work done because he was "bored" and did not even need it.
Robbie, based in Los Angeles, made his admission during the recording of the latest Graham Norton Show during a discussion about cosmetic surgery.
"I've lived in LA for a long time and they say, 'If you sit in a barber's shop for long enough you will get a hair cut'. Well, if you live in Los Angeles for long enough you're going to get some surgery.
"I've had a thatch done, I'll tell everyone now. I didn't even need it, that's the weirdest thing. I had like three months off and got bored so thought, I'll go in."
The star, who has recently been sporting a Pepe Le Pew-style streak in his hair, indicated the hair had been enhanced around the front of his hairline, rather than all over.
"I just got some at the sides and front done," Robbie told presenter Graham during the show, to be screened at 10.35pm on BBC One tonight (November 22).