Hundreds of helicopters and 400 firefighters were deployed to the south of France on Thursday as torrential rain caused heavy flash floods.
The worst-hit areas are Gard, Ardèche and Drôme.
Top Gard official Thierry Dousset told BFM-TV that about 750 people were evacuated from five campsites.
The local Gard government also said four German children were hospitalised for hypothermia in Bagnols-sur-Ceze.
BFM-TV reported that the missing person was a 70-year-old German man.
As much as four inches of rain fell in less than three hours in some areas of central and southern France.
Rachel Buchanan, of Oxford, was one of thousands of holidaymakers who had headed for campsites in Gard, Ardeche and Drome when heavy rain destroyed tents as the heatwave broke with a vengeance.
She woke up to discover her camper van was one foot deep in water and had to race to escape waters gushing over hillsides into the site.
Driving in our camper van was very frightening as the road by the river was completely under water
Ms Buchanan was left with a «frightening» drive through floodwaters that had hit a site near a river in the neighbouring Ardeche region.
She told the BBC: “The level and speed of the river today was extraordinary.
«We woke up in about a foot of water.
«Driving in our camper van was very frightening as the road by the river was completely under water and waterfalls had appeared from nowhere crashing down the gorge.»
Four German children had been hospitalised for hypothermia in Bagnols-Sur-Ceze, a town on the Ceze River.
More than 400 firefighters and police officers, many from other regions, helped with the evacuations, using helicopters to spot campsites and rescue people.
Many roads in the area were left cut off, with the authorities urged members of the public to be prepared for more disruption.