The missing man, a 70-year-old German citizen, was a monitor at one of the campsites who was feared to have been swept away by flood waters along with his van, BFM-TV reported.
Rescuers have evacuated 750 people, including many children and tourists, from campsites in southern France after torrential rain swelled rivers in the Gard region.
Rescuers walk past a damaged tent in a flooded camp ground in Saint-Julien-de-Peyrolas, southern France.
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 German pensioner remains missing after being swept away.
The worst-hit areas are Gard, Ardèche and Drôme.
He was the only person unaccounted for after a rescue operation at three campsites in the area, where a heatwave ended abruptly this week in storms that caused flash flooding.
Footage posted on social media showed dramatic scenes of flooding. We just have what we're wearing, ' Rita Mauersberger, a visitor from Germany who was among the campers taking shelter in a local hall in Saint-Julien-de-Peyrolas, told France Info radio.
More than 400 firefighters and police - as well as four helicopters - have been deployed.
A spokesman for the Gard Prefectures said: 'It is hoped that the flooding will reach its peak by around 11pm on Thursday'.
In one summer camp in Saint-Julien-de-Peyrolas, 119 children were evacuated.
Around 19,000 homes are without power, Le Monde reported.