Tsushima Island in the context of "Korea Strait"

The Saikaidō (西海道; Japanese pronunciation: [sai.kaꜜi.doː], lit. 'Western Sea Circuit') is a Japanese geographical term. It means both an ancient division of the country and the main road running through it. Saikaido was one of the main circuits of the Gokishichidō system, which was originally established during the Asuka period.

This name identified the geographic region of Kyūshū and the islands of Tsushima and Iki. It consisted of nine ancient provinces and two islands. The provinces included Chikuzen, Chikugo, Buzen, Bungo, Hizen, Higo, Hyūga, Satsuma and Ōsumi.

Nagasaki Prefecture (長崎県, Nagasaki-ken) is a prefecture of Japan, mainly located on the island of Kyūshū, although it also includes a number of islands off Kyūshū's northwest coast - including Tsushima and Iki. Nagasaki Prefecture has a population of 1,246,481 (1 February 2025) and has a geographic area of 4,130 km (1,594 sq mi). Nagasaki Prefecture borders Saga Prefecture to the northeast.

Nagasaki is the capital and largest city of Nagasaki Prefecture, with other major cities including Sasebo, Isahaya, and Ōmura. Nagasaki Prefecture is located in western Kyūshū with a territory consisting of many mainland peninsulas centered around Ōmura Bay, as well as islands and archipelagos including Tsushima and Iki in the Korea Strait and the Gotō Islands in the East China Sea. Nagasaki Prefecture is known for its century-long trading history with the Europeans and as the sole place of direct trade and exchange between Japan and the outside world during the Sakoku period. Nagasaki Prefecture is home to several of the Hidden Christian Sites in the Nagasaki Region which have been declared a UNESCO World Heritage Site.

Tsushima Strait (対馬海峡, Tsushima Kaikyō) or Eastern Channel (동수로; Dongsuro) is a channel of the Korea Strait, which lies between Korea and Japan, connecting the Sea of Japan, the Yellow Sea, and the East China Sea.

The strait is the channel to the east and southeast of Tsushima Island, with the Japanese islands of Honshu to the east and northeast, and Kyushu and the Gotō Islands to the south and southeast. It is narrowest south-east of Shimono-shima, the south end of Tsushima Island proper, constricted there by nearby Iki Island, which lies wholly in the strait near the tip of Honshu. South of that point Japan's Inland Sea mingles its waters through the narrow Kanmon Strait between Honshu and Kyushu, with those of the Eastern Channel, making for some of the busiest sea lanes in the world.

Iki Island (壱岐島, Iki-no-shima), or the Iki Archipelago (壱岐諸島, Iki-shotō), is an archipelago in the Tsushima Strait, which is administered as the city of Iki in Nagasaki Prefecture, Japan. The islands have a total area of 138.46 square kilometres (53.46 sq mi) with a total population of 28,008. Only four (4) of the twenty-three (23) named islands are permanently inhabited. Together with the neighboring islands of Tsushima, they are collectively within the borders of the Iki–Tsushima Quasi-National Park.