The incident occurred around 8 a.m. ET in the area of Highway 404 and Steeles Avenue. Police said a collision between a car and a small pickup truck resulted in the single-vehicle rollover.
The name of the driver has not been released by police. He remains in hospital in critical condition.
Police said earlier they believe speed may have been a contributing factor in the collision.
A stretch of Highway 404 southbound was closed for several hours while the OPP`s traffic reconstruction team investigated, but all lanes have since reopened.