Variability in habitat use influences animal distribution and can lead to different life-histories across sexes and individuals. Such differences are frequently observed in species with sexual dimorphism, but less known in monomorphic species. In this study, we investigated sex-related differences in the diet of the mountain hare (Lepus timidus) – a monomorphic species – during the mating season (spring) and the post-reproductive period (autumn). We investigated the diet composition by the microhistological analysis of pellets of 18 individuals (8 males, 10 females) which were surveyed from 2015 to 2017 by a non-invasive genetic population monitoring in the Swiss National Park. We found significant differences in diet composition between seasons and sexes. Females consumed a higher proportion of graminoids in the mating season, probably due to higher energetic needs at that time of the year. Our study shows that further attention should be given to sex-specific diet composition of monomorphic species.