<p>Studying the relationship between food availability and feeding behaviour reveals the feeding strategy of a species. We studied the dietary habits of the omnivorous raccoon dog (<i>Nyctereutes procyonoides</i>) in northern Japan, focusing on the relationships with availability of food items to determine their feeding strategy. We analysed faecal samples (<i>n</i> = 138) collected between 2020 and 2024 and evaluated their composition (percentage area and percentage frequency of occurrence). We also performed phenological surveys and pitfall trapping to evaluate the availability of fleshy fruit and terrestrial insects, respectively. Diets of raccoon dogs were composed of fleshy fruits (percentage area: 60%; percentage frequency of occurrence: 99%), insects (22% and 84%), and leaves (11% and 87%). The percentage of fruits was high and stable except in summer when the percentage of insects increased. The percentages of vertebrates (mammals and birds) and anthropogenic items were low but increased in winter and spring. We found (1) negative correlations between fruit availability and vertebrate feeding and dietary diversity; (2) a positive correlation between insect availability and feeding; and (3) a negative correlation between insect availability and feeding on anthropogenic items. These results imply that raccoon dogs employ an opportunistic feeding strategy that includes prey switching. In northern Japan, they preferred fleshy fruits and insects on the ground and consumed vertebrates and anthropogenic items as alternative diets in food-scarce seasons. The feeding plasticity of raccoon dogs may have enabled them to inhabit a variety of habitats in their natural range and where they were introduced (e.g., Europe). Our findings provide some insight into understanding their ecological role (e.g., seed dispersal) and inter-specific interactions with sympatric animals and plants (e.g., predator-prey relationship) of raccoon dogs in their natural range.</p>