Speaking to TMZ over the phone, the pop teen says he ran into a glass panel between songs, bumping his head hard.
The Bieb went on with the show, despite feeling dizzy, but as soon as he returned to his dressing room he passed out cold.
Justin suffered a concussion, but doctors checked him out and told him he'd be fine.
Amazingly, this is the third time he's smacked his head on a glass window or door. In two separate incidents in 2010, the teenager collided with a revolving glass door and a glass panel in the office of a US radio station.
"I guess me and glass windows just don't work," he joked to TMZ.
Here he is the first first time he collided with glass:
And the second time...