Tsushima Island is an island of the Japanese Archipelago situated in the middle of the Tsushima Strait at 34°25'N and 129°20'E. The main island of Tsushima was once a single island, but the island was divided into two in 1671 by the Ōfunakosiseto canal and into three in 1900 by the Manzekiseto canal. Tsushima is composed of North Tsushima Island, South Tsushima Islands (Shimo Jima), and over 100 smaller islands. Generally the name Tsushima refers to all the islands. The main three islands are the largest island of Nagasaki Prefecture and the sixth largest in Japan (excluding Honshu, Kyushu, Shikoku and Hokkaido). The city of Tsushima encompasses the entire island.
Tsushima Island is located west of the Kanmon Strait at a latitude between Honshū and Kyūshū of the Japanese mainland. The Tsushima Strait splits at the Tsushima Island Archipelago into two channels; the wider channel, also closer to the mainland of Japan, is the Tsushima Strait. Ōfunakoshi-Seto and Manzeki-Seto, the two canals built in 1671 and 1900 respectively, connect the deep indentation of Asō Bay to the east side of the island. The archipelago comprises over 100 smaller islets in addition to the main island.