The Baby singer's video message - which forms part of the 'It Gets Better Project' campaign – features Bieber urging bullying victims to tell an adult what is happening.
He says: 'There's nothing cool about being a bully.
'If you're getting bullied, make sure to tell someone and, you know, it gets better. And if you're a bystander, make sure to step in and, you know, help out.'
The anti-bullying project was launched after a spate of suicides of gay teenagers who had been bullied. Anne Hathaway, Katy Perry, Ke$ha, and US President Barack Obama have also recorded their own messages for the campaign.