Dogs have cohabited with humans since the Upper Paleolithic and their lifestyle and diet during late prehistory probably already depended on the role they played in past societies. Here, we used a combination of stable isotope analyses and three-dimensional geometric morphometrics to test for differences in, and associations between, diet and mandibular morphology based on 150 dogs of three sites of the Chalcolithic Gumelnit,a culture in Romania (4550-3900 cal. BCE) characterized by different socio-economic systems. At Ha<SIC>rs,ova-tell and Bordus,ani-Popina, where the subsistence economy is mainly based on herding, dogs have a variable diet that is rich in domestic animals (sheep, cattle, pig) and may contain fish. In contrast, at Vitanes,ti-Magurice, where hunting predominates, the diet of dogs is more specialized towards large game (red deer, aurochs, wild horse), reflecting the composition of human food refuse. Moreover, dogs have more robust (but not larger) mandibles at this site with shapes suggesting a greater importance of the temporal muscle important for the capture of large prey and the breaking of large bones. The strong covariation between mandible shape and stable isotope signatures suggests functional adaptations to diet. Overall, our results support the idea that prehistoric dogs adapted to human lifestyles.