The life-sized bust showing the Roman ruler as a balding and ageing man with wrinkles and hollows in his face is tentatively dated to 46 BC.
Among other items in the treasure trove of ancient objects found in the bed of the river was a 1.8m marble statue of Neptune, dated to the first decade of the third century after Christ.
Two smaller statues were also found, both in bronze and measuring 70cm each. One of them, a satyr with his hands tied behind his back, "doubtless" originated in Hellenic Greece, the French culture ministry said.