We aimed to ascertain the environmental determinants of the diets of Japanese macaques Macaca fuscata in relation to geographical variables (latitude and elevation) and environmental variables [annual rainfall, mean temperature, snow characteristics and normalized difference vegetation index (NDVI), a proxy for primary productivity] We collected dietary data from the entire range of Japanese macaques (29 study sites) and extracted each dietary component to determine spatial patterns over the species' range and between forest types. We then conducted model selection to identify the environmental determinants of staple diets and dietary diversity. Japanese macaques' diets were mainly composed of 1) foliage, 2) fruits and seeds, and 3) bark and leaf buds, throughout the entire range. Percentages of bark and buds were greater in deciduous forests than in evergreen forests; significant differences in the proportions of the other two major components and in dietary diversity were not observed in different forest types. Macaques inhabiting forests with higher NDVI and with less snow fed more on fruits and seeds, and a high dietary diversity was observed. In snowy forests, macaques inhabiting higher elevation fed more on foliage, while those inhabiting lower elevations fed mainly on bark and buds. When studying spatial patterns in the feeding behaviour of mammals, both environmental and geographical variables should be treated equally since the effect of environmental variables may be independent of the effect of geographical variables. In addition, more emphasis should be placed on snow characteristics, because snow cover leads to decreased availability of terrestrial food items and to increased energy costs (for locomotion and body temperature regulation), which can affect feeding behaviour and dietary diversity.