The rollover occurred Saturday afternoon in Clarington, Ont., about 80 kilometres east of Toronto.
Durham Regional Police say they were called to a stretch of Moffat Road, just south of Concession Road 4, at 4:44 p.m. Saturday.
The man was found partially pinned beneath the Chevrolet Silverado pickup truck that had overturned in a ditch.
He was taken to a local hospital and later transferred to a Toronto hospital, where police say he remains in critical condition.
Police say "weather and road conditions were favourable at the time of the collision" and they are investigating if alcohol was a factor.
Anyone with information about the collision is asked to contact police at 1-800-579-1520, ext. 5255.
Information can also be passed on anonymously through Crime Stoppers at 1-800-222-8477.