A total of 1,600 people, majority campers, were rescued across Gard and the nearby Ardeche and Drome regions, according to France's interior ministry.
BFM-TV said he was a 70-year-old German citizen serving as a monitor at the Saint-Julien-de-Peyrolas site and feared to have been swept away by floodwaters along with his van.
A large number of campers, many of them children, were evacuated in the Gard region as floodwaters destroyed tents and inundated streets.
Trapped by a muddy, fast-flowing torrent after the Ardeche river burst its banks he sought shelter inside his caravan, but the vehicle was swept away.
The Gard administration also said four German children were hospitalized on Thursday for hypothermia in Bagnols-sur-Ceze, a town on the Ceze River.
A top regional official, Thierry Dousset, told French news channel BFM TV that the some 750 people evacuated came from five campsites.
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The Gard is hugely popular with British holidaymakers in August, while many others from the United Kingdom have retired to the area, or have second homes there.
They were among 10 people treated for minor injuries.
About 400 firefighters and policemen, backed by four helicopters, helped in the evacuations and performed rescue operations, the French interior minister said.
Nearly 75mm of rain fell over the area in just 24 hours with more outbreaks possible during the next few days. Authorities warned that the flooding would take time to recede and urged people to be vigilant.
Several parts of central and southeast France are on flood alert after weeks of blazing temperatures erupted in storms on Thursday, turning several campsites into mudbaths.