Japan, Sea of, or East Sea, an arm of the Pacific Ocean. It lies between the islands of Japan and the Asiatic mainland (Korea and eastern Siberia). In the south, it is connected with the East China Sea by the Korea Strait. In the north, the Tsugaru Strait connects it with the Pacific, and La PĂ©rouse (or Soya) Strait and the Tatar Strait connect it with the Sea of Okhotsk. The Sea of Japan has an area of about 390,000 square miles (1,010,000km2).