Torrential rain struck the Alpes-Maritimes administrative department on Saturday evening, flooding local towns including Cannes and disrupting transport routes.
The victims included three people killed at a retirement home that was flooded in the village of Biot, and seven inhabitants of the town of Mandelieu-la-Napoule who had attempted to reach their cars in a garage, local government officials said.
Five people were still reported missing on Sunday afternoon, which could push the death toll higher, officials said.
In the resort town of Cannes, home to the prestigious annual film festival, two people were killed, the train station was flooded, and cars were swept along streets by torrents of rain.
Local rail services resumed on Sunday after being suspended overnight, holding up passengers including around 2,000 Italian pilgrims on their way back from Lourdes.
Train services, however, were expected to remain limited for the rest of Sunday, while some roads also continued to be blocked.
Thousands of homes were still without electricity on Sunday afternoon after as many as 70,000 suffered blackouts the previous evening, power network operator ERDF said.
Visiting the affected zone on Sunday, President Francois Hollande said the government would approve natural disaster status for the area at Wednesday's cabinet meeting and pledged relief payments to help cover damage within three months.
Residents were taken by surprise by the exceptionally violent downpour over Alpes-Maritimes, which lies at the eastern end of France's Mediterranean coast and borders Italy.
Cannes registered close to 180 millimetres (7.1 inches) of rain in three hours at the height of the storm, the interior ministry said.
Flights were diverted away from Nice airport on Saturday evening, leaving some passengers stranded overnight, officials said.
A soccer match between Nice and Nantes in the French first division was abandoned midway due to the rain, while hundreds of people were stranded overnight at a concert venue in Nice after attending a show by French rock legend Johnny Hallyday.
Hundreds of police and fire officers were mobilised for search and rescue operations across areas affected by the floods, while police were also drafted in to patrol shopping areas after reports of some looting overnight.
(Additional reporting by Gavin Jones in Rome, writing by Gus Trompiz; Editing by Raissa Kasolowsky and Susan Fenton)
