We report here on the isotopic analysis (carbon and nitrogen) of collagen extracted from a Neanderthal tooth and animal bone from the late Mousterian site of Jonzac (Charente-Maritime, France). This study was undertaken to test whether the isotopic evidence indicates that animal protein was the main source of dietary protein for this relatively late Neanderthal, as suggested by previous studies. This was of particular interest here because this is the first isotopic study of a relatively late Neanderthal associated with Mousterian of Acheulian Tradition (MTA, dating to approximately 55,000 to 40,000 BP) technology. We found that the Jonzac Neanderthal had isotopic values consistent with a diet in which the main protein sources were large herbivores, particularly bovids and horses. We also found evidence of different dietary niches between the Neanderthal and a hyena at the site, with the hyena consuming mainly reindeer.