Seasonal and short-term foraging patterns of the Tsushima marten (Martes melampustsuensis) were examined using the stomach content analysis. To do this, the indices of frequency, amount, and number of consumed individual animals and plants taken from 120 stomachs of roadkilled martens were investigated. The frequency of occurrence and the total wet weight and number of consumed individual insects, earthworms, and fruits represented 40–90% of all foraged food categories from spring to autumn. Thus, these three categories were regarded as the main foods of the Tsushima marten. However, martens did not forage on specific food categories intensively in winter. As the availability of the three main categories generally decreased in winter, martens expanded their target food categories and dietary niche breadth. The consumed wet weight of fruits was higher than that of the other main categories from summer to winter. As large fruits were available in these seasons, martens would obtain high food biomass over short periods efficiently. Quantitative stomach content analysis of the omnivorous Tsushima marten allowed us to reveal their seasonal different diet, the utilization of digestible foods (i.e., earthworms), and the influence of the fruiting period and size of each fruit on the short-term foraging efficiency.