To evaluate the seed shadow generated by wild Japanese martens (Martes melampus), we combined data on their ranging behavior obtained from the northern foot of Mt. Fuji, central Japan (7 males and 3 females) with data on gut passage time obtained from martens in Toyama Municipal Family Park Zoo (3 males and 1 female). The movement distances varied, and mean distances for 0–1, 2–3, and 4–5 h intervals were 152.4, 734.7, and 1,162.4 m, respectively, with no significant sex difference. The mean gut passage time of ingested seeds was 7.4 h (range: 0.6–51.7 h), and two-thirds were defecated within 12 h. Seeds of fleshy fruits were frequently transported 501–1,000 m, and 20% of ingested seeds were transported >1,000 m from the feeding sites. We found positive correlations between body size and home range of the animals in Japan and their seed dispersal distances. Compared to sympatric small-to middle-sized animals in forest (such as monkeys, raccoon dogs, and birds), their dispersal distances were much longer, although shorter than that of Japanese black bears. We concluded that Japanese martens are medium-range seed dispersal agents that could transport seeds from the source to open habitats, conducive for germination and/or growth, partly due to scent marking. We also would like to review corresponding data for other martens in America and Europe (M. americana and M. martes), and study the similarity and/or difference in relative importance of martens as seed dispersal agents.