Long-distance dispersal (LDD) is an essential event for species colonization and expansion in oceanic island ecosystems. Endozoochory by birds is an important factor promoting the LDD of plant seeds, but its contribution to interisland seed dispersal is still unclear. Here, we show possible seed dispersal by a seed predator pigeon, the Japanese wood pigeon Columba janthina, among oceanic islands in the Izu archipelago, Japan. Intact seeds were found in 44.5% of pigeon feces, indicating the contribution of these birds to seed dispersal. Seasonal population fluctuations and frequent movements between neighboring islands which are 4 km apart by pigeons can promote interisland seed dispersal. We found seeds from fruiting plants not located on the islands where the pigeon feces were collected, which could have been transported from another island. Although many tree species overlap their fruiting phenology with pigeon movement between islands, the seeds of only six species were dispersed by pigeons, and most of the dispersed seeds were from species with small seeds less than 1.5 mm in diameter. Seed consumption and frequent interisland movement of Japanese wood pigeons might homogenize the distribution of plants with small seeds among neighboring islands. In contrast, their intensive seed consumption might disturb the recruitment of plants with large seeds and make their expansion to other islands difficult. Such effects of mutualistic (dispersal) and antagonistic (predation) interactions with highly mobile seed predators may modify plant distributions and gene flow in oceanic archipelagos. We also found the increasing flying ability of pigeons by feather morphology and genetic structure on highly isolated island condition, despite the island rule of flightless bird evolution. The interisland movement of island pigeons may be adaptive to get food resources in isolated archipelagos with unstable habitat condition in each island. The diet analyses using DNA metabarcoding revealed the pigeons’ food preference, the seasonal shift of diet composition, and frequent consumption of introduced plants. The feeding strategies of pigeons in relation to the habit of interisland movement should be considered for their effective habitat conservation.