Dejima (出島^?, literally "exit island"; Portuguese: Dejima or Decima, Dutch: Desjima or Deshima or sometimes latinized as Decima or Dezima) was a small fan-shaped artificial island built in the bay of Nagasaki in 1634 by local merchants. This island, which was formed by digging a canal through a small peninsula, remained as the single place of direct trade and exchange between Japan and the outside world during the Edo period. Dejima was built to constrain foreign traders as part of sakoku, the self-imposed isolationist policy. Originally built to house Portuguese traders, it was used by the Dutch as a trading post from 1641 until 1853. Covering an area of 120 m x 75 m (9000 square meters, or 0.9 hectares), it later was integrated into the city.

"Dejima Dutch Trading Post" has been designated a Japanese national historic site.

In 1543 Portuguese traders were the first to land in Japan, on Tanegashima. Six years later the Jesuit missionary Francis Xavier landed in Kagoshima. At first Portuguese traders were based in Hirado, but they moved in search of a better port. In 1570 daimyo Ōmura Sumitada converted to Catholicism (choosing Bartolomeu as his Christian name) and made a deal with the Portuguese to develop Nagasaki; soon the port was open for trade. In 1580 Sumitada gave the jurisdiction of Nagasaki to the Jesuits, and the Portuguese obtained the de facto monopoly on the silk trade with China through Macau.