Ontario Provincial Police say they were called to the westbound lanes of Highway 401, east of Guelph Line at 12:36 p.m. — where a vehicle had rolled over and struck a tree.
The driver was pronounced dead at the scene. There was no immediate word on the driver's gender or age.
The westbound 401 lanes were closed at Highway 25 for the investigation, but had partially reopened by shortly before 2 p.m.
"We currently have OPP reconstruction teams at the scene conducting an investigation," said OPP spokesman Sgt. Kerry Schmidt.