The mayor of the village of Ramatuelle, which lies on the Gulf of Saint-Tropez, told the AFP news agency that 16 kilometres of coastline had been affected by the spill, including the beach where a bikini-clad Brigitte Bardot posed for scenes in the 1956 classic "And God created woman".
The pollution had affected "creeks and beaches, particularly banks of seagrass", Roland Bruno said.
Photographs on the website of the local Nice Matin paper showed a beach dotted with oily black clumps.
Regional maritime authorities said the oil appeared to have come from the spill triggered by a collision between two ships in the Mediterranean nine days ago.
Some 600 tonnes of bunker fuel leaked from the Cyprus-registered "Virginia" after it was rammed by a Tunisian freighter some 30 kilometres off the northern tip of the French island of Corsica on 7 October.
Officials said most of the spill had been cleaned up, but that some of the residue had become trapped in seagrass that washed up ashore in Saint-Tropez.
All the affected beaches will be closed during the clean-up, the municipality of Ramatuelle said on its website.