A second man disappeared while out on his boat and 4,000 homes have been left without power after the inundations in the department of Var, they said.
The victim, a 73-year-old man, died in his basement, local official Laurent Cayrel told a press conference. He added that there had been "record rainfall".
"The situation is very worrying, and it's not going to get any better tonight (Sunday)," he said, with forecasts of more rainfall.
Around 155 people had been airlifted from the worst-hit areas and efforts to evacuate residents by boat were continuing.
"It was really staggering," she said. "We had called the fire brigade who told us to move the furniture upstairs.... We had very little time, the house flooded very quickly.