South Korea South Korea, officially known as the Republic of Korea, country in northeastern Asia that occupies the southern portion of the Korean Peninsula. South Korea is bounded on the north by North Korea; on the east by the Sea of Japan; on the southeast and south by the Korea Strait, which separates it from Japan; and on the west by the Yellow Sea. It has a total area of about 38,023 sq. mi., including numerous offshore islands in the south and west, the largest of which is Cheju (area, 1829 sq. km/706 sq. mi.). The state of South Korea was established in 1948 following the post-World War II partitioning of the peninsula between the occupying forces of the United States in the south and the Union