Local administration officials in the Var region evacuated some residents and urged others to stay indoors until the floodwater recedes.
They said a 73-year-old man was killed in his basement and another man died when his car was washed away. Another man, who was last seen on his boat, is missing.
Dozens of roads were closed across the region, which includes resort towns such as Saint-Tropez and villages in Provence.
Up to 8ins of rain was recorded over the weekend at some points, and rivers and streams that feed the Mediterranean flowed into residential areas.
Prime Minister Jean-Marc Ayrault visited the region yesterday to check on clear-up efforts.
Some 4000 homes have been left without power and local official Laurent Cayrel told a news conference there had been record rainfall.