A 70-year-old German man who was helping to supervise children at a summer camp at Saint-Julien-de-Peyrolas in Gard is feared to have been swept away by floodwaters, together with with his van.
Almost 120 children were evacuated from the site as emergency teams helped around 750 people to safety across the Gard region.
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.
Heavy floods have forced the evacuation of 1600 people - a lot of them campers - in southern France, officials say.
A search was underway for him.
Ten people, including four German children, have been taken to hospital with hypothermia and minor injuries, according to regional police spokesman Jérôme Tallaron. Five campsites in the area have been evacuated'.
The worst hit areas are Gard, Ardèche and Drôme, which are all popular holiday sites.
He added: 'Helicopters are being used in the relief operations, with valleys particularly badly hit'.
A smashed caravan after hitting a tree
Footage posted on social media showed dramatic scenes of flooding.
'No one has suitcases.
Rescuers walk past a damaged tent in a flooded camp ground in Saint-Julien-de-Peyrolas, southern France.
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.
Hundreds of firefighters backed by four helicopters helped in the evacuation. Authorities warned that the flooding would take time to recede and urged people to be vigilant.
In one summer camp in Saint-Julien-de-Peyrolas, 119 children were evacuated.
Several parts of central and southeast France are on flood alert after a blazing heatwave ended abruptly in storms on Thursday. Around 17,000 homes in the southwest and northeast were without power.