The two-meter long femur is thought to have belonged to a sauropod, a herbivorous dinosaur with a long neck and tail which existed more than 140 million years ago.
The bone was found in a thick layer of clay at the Angeac-Charente site in southwestern France earlier this week.
French dinosaur find
"This is a major discovery," Ronan Allain, a palaeontologist at the National Natural History Museum of Paris, told Reuters News Agency.
"These are animals that probably weighed 40 to 50 tonnes."
Allain told France's Le Parisien newspaper that, as well as the bone's size, its state of preservation was also impressive.
"We can see the insertions of muscles and tendons and scars. This is rare for big pieces which tend to collapse in on themselves and fragment."