The OPP say just before 4 a.m., the SUV and truck collided in the eastbound express lanes of the 401 east of Dixie Street.
Sgt.  Kerry Schmidt said the SUV hit the back of the tractor trailer.
A female passenger was pronounced dead at the scene, while a male driver of the vehicle was rushed to hospital in critical condition.
Schmidt said the eastbound express lanes are closed at Mississauga Road to allow for the cleanup and investigation.
The ramp from Highway 403 to the eastbound express lanes of Highway 401 is also closed.
Schmidt said motorists should expect the area to remain closed for much of the morning.