Using fecal analyses, we examined the food habits of three large herbivorous mammals, the sika deer (Cervus nippon), the Japanese serow (Capricornis crispus), and the wild boar (Sus scrofa) on Mt. Asama, central Japan. Ten fecal samples were collected in all four seasons during 2012–2013, although boar feces were not available in summer because of high-level dung beetle activity. The proportions of each food category were affected by both animal species and season. Wild boar feces had a unique composition, containing more underground plant parts and supporting organs compared with the feces of the two ruminants. Fecal samples from the sika deer were characterized by the predominance of dwarf bamboos. The fecal composition of the serow was similar to that of the deer, but contained fewer graminoids and more dicot leaves. Similarities in fecal compositions were often evident between the deer and the serow, but not between the ruminants and the boar. In the deer and the serow, the fecal particle sizes were of smaller dimensions than those of the boar, presumably reflecting morphophysiological differences. This is the first study to compare the diets of three sympatric herbivorous mammals in Japan.