Tsushima is an island city grouped in Nagasaki Prefecture, Japan. It is the only city of Tsushima Subprefecture and it encompasses all of Tsushima Island, which lies in the Tsushima Strait north of Nagasaki on the western side of Kyushu, the southernmost mainland island of Japan. As of March 2017, the city has an estimated population of 31,550 and a population density of 45 persons per km2. Its total area is 708.61 km2, 17.3% of the area of Nagasaki Prefecture.

Tsushima Island is an island of the Japanese archipelago situated in-between the Tsushima Strait and Korea Strait, approximately halfway between Kyushu and the Korean Peninsula. The main island of Tsushima, once a single island, was divided into two in 1671 by the Ōfunakoshiseto canal and into three in 1900 by the Manzekiseto canal. These canals were driven through isthmuses in the center of the island, forming "North Tsushima Island" (Kamino-shima) and "South Tsushima Island" (Shimono-shima). Tsushima also incorporates over 100 smaller islands, many tiny. The name Tsushima generally refers to all the islands of the Tsushima archipelago collectively. Administratively, Tsushima Island is in Nagasaki Prefecture.