Whilst marine resources are one of the pillars of the Mediterranean diet, their mode of acquisition and subsequent consumption by medieval populations in southern France are still not well known. Throughout Europe, bioarchaeological techniques, however, are beginning to reveal hitherto unknown aspects of these practices both dating to the medieval period as well as other periods of history and prehistory. This study involved the stable isotope analysis of five marine and catadromous taxa from three medieval sites in Provence, France: "rue Frédéric Mistral" at Fos-sur-Mer, "le Château" at Hyères and "Couvent des Dominicaines - Parking/Collège Mignet" at Aix-en-Provence. In total, 127 specimens, including Anguilla anguilla, Dicentrarchus labrax, Sparus aurata, Diplodus sargus and Mugilidae were subjected to carbon and nitrogen stable isotope analysis. The study provides a crucial and unprecedented point of reference of the carbon and nitrogen isotopic variability of one of the main dietary resources in the Mediterranean world, fish.