We investigated for the first time the prevalence of avian haemosporidia of genera Plasmodium, Haemoproteus and Leucocytozoon among birds and mosquitoes on Tsushima Island of Japan, which is located between Japan and the Korean Peninsula. Of 55 wild birds belonging to 33 species, 16 (29.1%) tested positive for haemosporidia as follows: Plasmodium spp. (11/55; 20.0%); Haemoproteus spp. (2/55; 3.6%); and Leucocytozoon spp. (3/55; 5.5%). A genetic lineage isolated from the Eurasian Sparrowhawk (Accipiter nisus) was identical to that of the known avian malaria parasite P. circumflexum. Several genetic lineages were identical or closely related to the parasite lineages that were previously detected in birds and mosquitoes in Japan and Korea. Another single identical genetic lineage was also detected in both migratory and resident birds. A total of 753 mosquitoes from 12 species were collected; and one fully fed Aedes albopictus was positive for avian Plasmodium(1/753; 0.13%) which is identical to a genetic lineage detected in both mosquitoes in Japan and birds in Korea. Blood-meal identifications of blood-fed mosquitoes showed direct contact between the mosquitoes and 4 species of mammals including humans, cattle, rodents and the endangered Tsushima leopard cat (Prionailurus bengalensis euptilura). Migratory birds use Tsushima Island as a site for wintering, breeding and resting, and our results suggest the transmission of avian haematozoa between resident and migratory birds during their stay on Tsushima Island.