Photo: Valais police
The accident happened at around 8.30pm on Saturday, Valais police said in a statement.
The car, registered in the southern canton, was being driven by an 18-year-old towards Sion when it left the road on a left-hand bend just after the village of Brignon.
The car hurtled down a slope for around 100 metres before crashing into a tree.
A 15-year-old passenger died at the scene.
The driver and two other passengers – all aged 18 – were taken to hospital in Sion.
Police have opened an investigation to determine the cause of the accident.