The pop star was with rumored girlfriend Selena Gomez. They had just finished celebrating his 17th birthday Tuesday night at a local restaurant when he made the obscene hand gesture.
The teen later took to his Twitter account to apologize for the incident.
"Had a great bday and at the end of the night we got surrounded by paps and I reacted in a way I know better. I'm sorry," Bieber tweeted on Wednesday. "It's not always easy but I know better than to react in anger."
Also on Wednesday, the pop sensation's hair was sold on eBay for $40,668. All of the money is being donated to an animal organization.
Bieber and Gomez have been rumored to be dating over the last few months, but the teens have not confirmed a relationship. On Sunday, they attended Vanity Fair's Oscar after-party together.