Ecological features and conservation requirements of populations at the latitudinal limits of a species’ geographical range frequently differ from those in other parts of the range. Identifying such differences is key to implementing effective conservation strategies for threatened range-edge populations especially, in the context of rapid global warming, at the lower-latitude range edge. We studied habitat selection and diet of the endangered Cantabrian Capercaillie Tetrao urogallus cantabricus in a recently discovered population at the southernmost edge of the sub-species’ range. This is the only Western Capercaillie population in the Mediterranean biogeographical region. We combined non-systematic surveys based on questionnaires, reports and field sampling with data from radiotracking to assess habitat selection. Diet was surveyed by micro-histological methods from droppings collected in the new population, which inhabits Pyrenean Oak Quercus pyrenaica forests and Scots Pine Pinus sylvestris plantations, and in two Cantabrian populations inhabiting Eurosiberian forests. Capercaillie preferred large (> 500 ha) and medium-sized (100–500 ha) Pyrenean Oak forest fragments and large Scots Pine plantations. Forest fragments smaller than 100 ha and non-forested habitats were always avoided. Diet differed markedly between Mediterranean and Eurosiberian populations. Bilberry Vaccinium myrtillus is common in the diet of most Capercaillie populations but was scarce in the study area and so was rare in the diet of the new population. Instead, Rockrose Halimium lasianthum was described for the first time as a major food resource for the Capercaillie and was consumed in autumn and winter. Pine needles were also heavily consumed in winter. We document for the first time the strong preference of Capercaillie for Pyrenean Oak forests and a moderately high consumption of the leaves, buds and acorns of this tree species throughout the year. Habitat selection and diet of this Mediterranean population differ from those of the core Cantabrian and other populations. Our results suggest a wider environmental tolerance (phenotypic plasticity) in the species than previously recognized. We advocate specific protection for this unique range-edge Capercaillie population and its Pyrenean Oak forest habitat.