Saiki, city, Ōita ken (prefecture), Kyushu, Japan, facing Saiki Bay. It developed as a castle town on the small delta of the Banjō River during the Muromachi era (1338–1573) and came into the possession of the Mori daimyo family in 1601. Because of its good harbour, Saiki was selected for a base of the Imperial Japanese Navy in 1933. After World War II most of the military facilities were converted to industrial use; ships, paper pulp, and cement are produced. The port has been engaged in foreign trade since the 1950s. Pop. (2005) 80,297; (2010) 76,951.