Up to 40,000 homes were without power, 1,000 people were evacuated, and several bridges were swept away as torrential rain flooded large areas of the Gard department.
The Herault and Vaucluse departments were also hit.
Hundreds of troops were being sent to the area to help, said the AP news agency. The 300 soldiers were reportedly joining forces with up to 500 other rescuers.
More severe rainstorms were forecast for Monday night, moving towards Marseilles.
Local authorities declared a state of alert as the flood waters rose.
Most of the dead came from the areas around Avignon, Nimes and Uzes. The town of Orange was badly flooded.
Vineyards producing Chateauneuf-du-Pape wine were also deluged.
Two motorways, the A9 and A7, had to be closed, leaving drivers with no other routes to and from the Mediterranean. Officials appealed to motorists to stay at home.
"We strongly urge people not to drive in the Gard and Vaucluse departments," said a spokesman for the Regional Traffic Information Centre (CRIR).
Rail travel was also severely disrupted. The high-speed TGV service was suspended when a tree fell onto the line. Rail travellers to south-eastern France were urged to postpone their journeys.
"The situation remains very alarming," said a police spokesman in Nimes.
Emergency services faced high-risk situations as they tried to deal with the cascading waters.
Some rescue vehicles were themselves swept away, and a firefighter was left critically ill after rescuing a couple trapped in their car in the village of Galargues in Herault.
The floods come only weeks after large parts of central and eastern Europe suffered their worst flooding in decades, killing dozens and causing devastation in many historic towns and cities.