The downpour hit the Alpes-Maritimes region, which lies at the eastern end of France's Mediterranean coast and borders Italy, on Saturday evening.
Eight people have been found dead and another five are missing, the government's local office said. President Francois Hollande had earlier said there had been at least 12 deaths.
The first victims were three people killed at a retirement home that was flooded in Biot, local officials said.
In the resort town of Cannes, which hosts the annual film festival, the train station was flooded, leading to the interruption of local rail services, while cars were swept along streets by torrents.
The flooding led to the closure of a local stretch of the A8 motorway and left thousands of homes without electricity.
In Nice, a soccer match between the town's team and Nantes for the French first division was abandoned midway due to the rain, while hundreds of people were stranded overnight at a concert venue after attending a show by French rock legend Johnny Hallyday. — Reuters
