-- What happens in Vegas doesn't always stay there.
The pop star, whose actual birthday was March 1, arrived after a long day of filming his upcoming Comedy Central special, where he was roasted by comedians Kevin Hart and Will Ferrell among others.
"I just had a long day, I'm pretty tired but I'm ready to turn up," the singer, dressed in black and white, said from the red carpet. "We're at Omnia nightclub, we're in Vegas, I'm 21 years old. And I'm ready to go out and have an awesome time tonight."
From the looks of things, he did.
Bieber made his way through the club's main room to the DJ booth, where he joined friend and world renowned DJ Martin Garrix, who was celebrating his 18th birthday that night.
"Put your hands up in the air," Bieber yelled out to the exuberant crowd as hundreds of flashes went off. "Everybody scream."
He then celebrated with an extravagant birthday cake that was a replica of his recent birthday present, a 1965 Lincoln Continental. As confetti burst into the air, Bieber jokingly swapped the numbers on the cake to make "12" instead of "21."
Later, he gave fans what they were hoping for, an impromptu performance, when he jumped atop the DJ booth to show off his dance moves and sing along to his track "Wait for a Minute."