A cross-country vehicle crashed off the road shortly after midnight (2200 GMT Tuesday) and fell about 200 metres, the fire brigade said.
The Carabinieri Police were investigating the cause of the accident in the province of Cuneo.
Of the nine people riding in the vehicle, 5 died at the scene, including the 24-year-old driver.
Four persons aged 17 to 24 were seriously injured and taken to hospital.
According to initial investigations, the vehicle was only meant to carry six people.
Local media and news agency, Ansa, reported the group of youths had been on their way to watch shooting stars during the Perseid meteor shower.
The accident reportedly happened at an altitude of 1,800 metres.