sea.
The Sea of Japan
The Korea Strait separates Korea and Japan. It lies between the Japanese island of Kyushu and the South Korean mainland. It is an important waterway for maritime transportation between the two countries.
The East Sea.
The Sea of Japan.
The Korean Strait
The islands of Japan are situated between the Sea of Japan and the Pacific Ocean. The Sea of Japan separates Japan from South Korea, North Korea, and Russia.
The small stretch of ocean that separates Japan from North and South Korea is called the Sea of Japan. The Sea of Japan also borders Russia.
The Korea Strait separates Japan and Korea. It connects the East China Sea to the Sea of Japan.
The Sea of Japan separates the island of Japan from mainland Korea.
The Korea Strait.
-- "The Yellow Sea"In response to the previous answer, the Yellow Sea separates The Korean Peninsula and Mainland China.The 'Sea of Japan' separates Japan from the Asian mainland (including the Korean Peninsula). The 'Korean Strait' also separates the southern coast of South Korea from Japan.Please see a labelled map in the related links below.
Russia. The distance between the island of Tsushima, Japan and the coast of Korea is an estimated 65 km (about 40 miles). The Soya Strait, which separates Sakhalin Island, Russia from Hokkaido, Japan is about 40 km wide (25 miles).