OPP Const. Graham Williamson told CBC News that the crash occurred at around 3 p.m. when the westbound vehicle went off the road just west of Highway 6, in the Township of Puslinch.
Williamson said the vehicle went into the ditch, came back out of the ditch, travelled across the westbound lanes of traffic and stopped when it struck the centre median.
The woman who died was sitting in one of the back seats in the vehicle at the time of the crash. Her name has not been released.
Williamson said the crash remains under investigation.