In this paper we performed stable isotope analysis on bone collagen of 81 samples from at least 39 brown bears (Ursus arctos) dating from Late Pleistocene to nowadays, that lived in the western of the Cantabrian Mountains. To interpret the data obtained we compared brown bear stable isotope signatures with those of cave bears (Ursus spelaeus), red deer (Cervus elaphus), and humans (Homo sapiens) from the same area. We observed that the diet of cave bears and brown bears in the Cantabrian Mountains was based on vegetable matter, although their different isotopic signatures suggest different ecological niches: wooded lowlands for the cave bear and steep highlands with scarce tree cover for the brown bear. In the Holocene brown bear maintains isotopic signatures similar to the Pleistocene ones in spite of the climatic tempering, which seems to be related to an even greater displacement toward the uplands due to a greater anthropic pressure in the ecosystem.