Mustelids are omnivorous–frugivorous and exhibit temporal dietary switching and specialism; however, the underlying mechanisms are unclear. We studied the feeding strategy of the Japanese marten (Martes melampus) in northern Japan by examining the relationship between dietary composition and abundance of main dietary items (fruits, mammals, and arthropods). The percentage of fruits in fecal samples over the study period (2020–2022) was much higher (65% in terms of the area percentage of the point-frame method) than that of arthropods (13%, with a higher proportion in spring) and mammals (11%, with a higher proportion in summer). The percentage of fruits in marten diets increased when abundance of arthropods and mammals was lower, which implies that Japanese martens are opportunistic feeders while being fruit-dependent. Different from other Martes species in boreal regions that depend more on small mammals, Japanese martens use a dietary switching strategy and prefer to eat fruits when they are seasonally available. The long-term variation in marten food habits implies that the ecological role of martens, such as controlling populations of prey animals and seed dispersal agents, can change transiently; therefore, monitoring the food environment is necessary to address inter-specific interactions in forest ecosystems.