Hail, heavy rain and winds peaking at 224 km/h swept the island early in the day. Two of the victims were killed when trees fell in campsites.
Storms formed at sea will affect large parts of the western Corsica coast throughout the night from Thursday to Friday, Météo-France said.
French Interior Minister Gérald Darmanin said 350 people had gone missing at one point when cruise boats capsized or drifted, but it has now been confirmed that they are alive.