In antiquity the department was inhabited by the Santones, a Gaulish speaking tribe, who marched with the Pictones (or Pictavi) in the north, the Lemovices, and Petrocorii in the east, and the Bituriges Vivisci in the south. Although placed in Upper Aquitaine by the Roman admistrators, it is certain that the Aquitani with their non-Indo-European language did not extend this far north. The oldest strata of habitation names should then be Gaulish.

There is no evidence for a non-Indo-European substrate. As Latin displaced Gaulish and developed into Old French, the inhabitants of Charente Maritime used the Poitevin and Saintongeais dialects of the Langue d'Oil. During the transition period the language of the area was enriched by Germanic loan words linked to pirate raids on the coast and up the navigable rivers. Some of these remain fossilied in the region's place-names. Frankish and Gothic landowners have also left their traces notably in Aunis (Agiloneus, land of a man with the German name *Agilon, hence Chatel|aillon and Agiloneus became Alnis hence Aunis).