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 top Gard official, Thierry Dousset, told BFM-TV about 750 people were evacuated from five campsites.
Rescuers believe the man sought shelter in his camper when fast-flowing water burst the banks of the nearby L'Ardeche River, but the vehicle was swept away.
Hundreds of rescuers backed by helicopters moved out about 1,600 people, a lot of them campers, in three regions of southern France where heavy rain caused flash flooding and transformed rivers and streams into torrents, the interior minister said Thursday.More news: Sacha Baron Cohen's interview with Sheriff Joe Arpaio is absolutely nuts
A search was underway for him.
The Gard government also said four German children were taken to hospital on Thursday suffering with hypothermia in Bagnols-sur-Ceze.
Footage posted on social media showed dramatic scenes of flooding.
About 400 firefighters and policemen, backed by four helicopters, helped in the evacuations and performed rescue operations, the French interior minister said.More news: Manchester United vs Leicester City
Tents have been destroyed and roads overwhelmed with water after nearly three inches of rain fell in under 24 hours, causing rivers to overflow at a rapid rate. A sizzling heatwave across Spain has left 10 people dead in a week.
Pictures showed rescuers wading through flooded campsites with damage to tents, caravans and bicycles.
The French weather service has issued orange and yellow alerts for parts of southeast France that will remain in place until 4pm.
Electricity supplies have been cut in some villages, while roads have been gridlocked because of the number of people trying to get to high ground.More news: North Korea threatens to stall denuclearization in warning to US
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