Here we report the strong decline of a population of the endangered species Parnassius mnemosyne (Linnaeus, 1758) (Lepidoptera: Papilionidae) in the National Park of Foreste Casentinesi, Monte Falterona and Campigna (Italy). We compared historical presence data (before 1969) with current data from two years of monitoring (2018–2019) and provided evidence of a drastic reduction in the number of sites inhabited by the species. A preliminary assessment suggested that the population of P. mmemosyne occurring in the Park is limited to a few individuals. We argue that the population of this iconic and charismatic butterfly is at the verge of extinction in this National Park, probably because of a combination of habitat loss (i.e. decrease in size and number of areas of open grassland on the mountain belt) and climatic changes. Being one of the few populations of P. mnemosyne in the Northern Apennines, the implementation of protection measures is a high conservation priority. Several other butterfly species on the Italian mountains that are facing the same survival challenges would also benefit from the establishment of conservation actions aimed at improving habitat quality for P. mnemosyne. Present article is meant to call for action researchers, stakeholders, and especially decision-makers in order to increase the efforts to upturn the evident decline in abundance of this population. Key words: biodiversity conservation, butterflies, habitat loss, habitat management, Lepidoptera, Parnassinae