Sado (säˈdō) [key], island, 330 sq mi (855 sq km), in the Sea of Japan, off the west coast of N Honshu, Japan. Mt. Kimpoku (3,872 ft/1,180 m) is the highest point. The fertile central lowlands are an important rice-growing region; fishing and tourism are also important. Aikawa, the administrative center of the island, is the site of gold and silver mines which have been worked since 1601. In feudal times the Emperor Juntoku and the priest Nichiren were exiled there.