The Sea of Japan or Japan Sea in most countries and the United Nations and in academic fields, known as the East Sea in South Korea, the East Sea of Korea in North Korea, and the Japan Sea in China, is a marginal sea of the western Pacific Ocean, bound by the Japanese islands of Hokkaido, Honshu and Kyushu and Sakhalin island to the east, and the Korean peninsula and Russia to the west.

The Sea is connected to other seas by six shallow straits: the Strait of Tartary or Mamiya Strait between the Asian mainland and Sakhalin; La Perouse Strait or Soya Strait between the islands of Sakhalin and Hokkaido; the Tsugaru Strait between the islands of Hokkaido and Honshu; the Kanmon Strait between the islands of Honshu and Kyushu; the Tsushima Strait between the islands of Kyushu and Tsushima; and the Korea Strait between Tsushima and the Korean peninsula.

The deepest point is 3742 meters below sea level, its mean depth is 1752 metres. The surface area of the Sea is about 978,000 km². The Sea has three major basins. The Yamato Basin lies in the south east; the Japan Basin in the north; the Tsushima Basin in the south west. The Japan basin is the deepest area of the Sea, and the shallowest water can be found in the Tsushima Basin.

On the eastern shores, the continental shelves of the Sea are wide, but on the western shores, particularly along the Korean coast, they are narrow, averaging at about 30 kilometres. The warm water in the Sea contributes to the mild climate in Japan.

The areas in the north and the south east are rich fishing grounds. The importance of the fishery in the Sea is well illustrated by the mutual claims on the Liancourt Rocks by Japan and South Korea. The Sea is also important for its mineral deposits, particularly magnetite sands. There are also natural gas and a few petroleum fields. Since the growing of the East Asian economies, the Sea is an important commercial waterway.

The name of this water is challenged by the two Koreas. Japan insists that it is called Sea of Japan, which also is the international de facto standard. However, the governments of North and South Korea challenge this name, contending it is a symbol of Japan's colonial and imperialistic past in the region. The South Korean government wants the name East Sea to be used, while the North Korean government prefers East Sea of Korea. In China the sea is known as Sea of Japan, as "East Sea" means East China Sea.

See also: Dispute over naming the body of water between Japan and the Koreas