The sea between Korea and Japan was referred to as the Sea of Joseon (Korea) even in Japan from the 17th century to the later part of the 19th century. According to the "The Study on Sea of Japan" published in 1975 by a Japanese scholar, Kasuno, it was not until 1815 that Westerners started to make reference to the Sea of Japan. The sea was called in Japan by various names, including West Sea of Japan, Tartar Sea, or Sea of Joseon (Korea), until the 19th century.

There is indisputable evidence that many Japanese maps published during the 19th century made references to the sea as the Sea of Joseon (Korea). The maps defining the sea as the Sea of Joseon (Korea) include "Map of Asia" drawn by Katsuragawa in 1794, "The Map" by Takahashi in 1809, "World Map (Chikyu Bankoku Hozu)" in 1853, and "New World Map" by Kisaku in 1897. In addition to these old maps, "Collection of Korean Treaties," which contains diplomatic documents concluded between Korea and Japan in 1883, also describes the sea as the Sea of Joseon (Korea).

Source: "East Sea; The Name EAST SEA Used for Two Millennia", April 2003