A man and a woman were riding a motorcycle when the driver lost control at around 1 a.m. Sunday.
Provincial police spokesperson Daniel Thibaudeau said the motorcycle was traveling westbound in the tunnel.
"It lost control, hit the wall on the left side and ended up a few metres down the roadway," he said.
The passenger, in her fifties, was pronounced dead on the scene.
The man driving the motorcycle was taken to hospital with serious injuries including multiple fractures.
Thibaudeau said speed might have been a factor in the accident.