Bieber, 18, expressed his horror at William's hairline when he was asked in an interview what he would do if his own famous locks started thinning, reported Daily Express.
"I mean, there are things to prevent that nowadays, like Propecia. I don't know why he doesn't just get those things, those products. You just take Propecia and your hair grows back," Bieber said.
The Prince is also known for his good-humoured approach to his thinning hair, accepting his hair loss.