Speaking in the latest episode of his YouTube series, the singer admitted he became addicted to taking pills and smoking cannabis when he was 13.
"People don't know how serious it got," he said in the episode The Dark Season.
"I was waking up in the morning and the first thing I was doing is popping pills and smoking a blunt and starting my day."
Last year, the 25-year-old Canadian took a break from music to repair "some of the deep-rooted issues", but announced before Christmas he will return in 2020 with a new album and US tour.
He has also previously opened up about his heavy drug use and abusing his relationships, but this latest video is the first time he has spoken about it on camera.
In the latest episode of his Justin Bieber: Seasons series, the performer said at his lowest point he feared his dependence on drugs would kill him. After binging on ecstasy and hallucinogenic mushrooms, Bieber say members of his security team would have to check his pulse throughout the night to make sure he was still breathing.
His struggles to cope with the pressures of fame led him on to harder drugs.
"I was young, like everybody in the industry and people in the world who experiment and do normal, growing up things," he explained.
"But my experience was in front of cameras and I had a different level of exposure. I had a lot of money and a lot of things."
Bieber married model Hailey in 2018, who said in the documentary that she chose not to get involved in his life until he "chose to get sober".
The singer has had several brushes with the law, including being charged with assault and careless driving.
The feature also showed how the star uses an oxygen chamber to help relieve stress, anxiety and depression, and how he works closely with a brain disorder specialist.