Police said that a sedan travelling westbound on Highway 401 entered the centre median and struck a cement culvert at around 2:10 p.m., causing the car to rollover.
Robert Smart, 61, was a passenger in the car and was pronounced dead at the scene. The driver, Lynda Baynton, 65, was airlifted to hospital but succumbed to her injuries.
Ontario Provincial Police are still investigating what may have caused the driver to swerve into the median.
The incident closed the westbound passing lane for nearly eight hours, with the highway fully reopening at around 10 p.m. Saturday evening.