The sea between Japan and Korea has two official names
Japan's government calls it the Sea of Japan. South Korea's government calls it the East Sea. Both names are in real, current, official use — and international mapmakers are stuck in the middle.
Sea of Japan
The name used in Japan's official government communications and the predominant name in international cartography since it became standard in Western maps in the 19th century.
East Sea
South Korea argues "Sea of Japan" only became dominant during Japan's 1910–1945 colonial rule over Korea, when Korea had no voice internationally — and that "East Sea" was used for centuries before that. North Korea prefers "Korean East Sea."
Neither government recognizes the other's name. South Korea has petitioned the International Hydrographic Organization — the body that sets global standards for naming seas — to adopt "East Sea" or both names together. Japan maintains "Sea of Japan" is the established, uncontested standard. No resolution both governments accept has been reached.
In practice, most international maps and atlases still default to "Sea of Japan," though some now print both names, and South Korean and North Korean sources use "East Sea" exclusively.
Sources: Government position statements from Japan's Ministry of Foreign Affairs and South Korea's Ministry of Foreign Affairs; International Hydrographic Organization records. Country shapes: Natural Earth.