The 'As Long As You Love Me' singer questioned why Prince William has yet to undergo treatment to help restore his hair in a recent interview with Rollercoaster.
"I mean, there are things to prevent that nowadays, like Propecia," Bieber told the magazine, reports the Daily Telegraph.
"I don't know why he doesn't just get those things, those products. You just take Propecia and your hair grows back. Have you not got it over here?"
In 2010, a spokesperson for hair loss company Rogaine expressed interest in using Prince William as the face of the brand.
Jennifer Aniston defended the future king's hair loss last year, saying: "I think he should just let whatever's happening happen. Don't you?"