The 18-year-old recording star told the celebrity news Web site he was disoriented but went back on stage and finished his set after Thursday's mishap.
When the show was over, he walked to his dressing room and fell unconscious for about 15 seconds, he said.
TMZ quoted doctors as saying Bieber suffered a concussion but is expected to be fine.
"Just smacked my head and needed some water. All good," the recording artist said via Twitter Thursday. "I'm Canadian. We are tough. LOL. It's all good. Just gotta take it easy the rest of the night. Back at it again for you guys tomorrow. Thanks."