The Ariake Sea (有明海, Ariake-kai) is a body of salt water surrounded by Fukuoka, Saga, Nagasaki, and Kumamoto Prefectures, all of which lie on the island of Kyūshū in Japan. It is the largest bay in Kyūshū. Its deepest point is only about 20 meters deep, and extreme tides exceed 4 meters (13 ft), covering roughly 1,700 square kilometres (660 sq mi). Isahaya Bay is a branch of the Ariake Sea. Across the Amakusa Islands lies the Yatsushiro Sea.
Many harbors are located on the coast of the Ariake Sea. Among them are Misumi (in the city of Uki, Kumamoto Prefecture), Shimabara (Shimabara, Nagasaki), Taira (Unzen, Nagasaki), Nagasu (Nagasu, Kumamoto), Kumamoto (Kumamoto, Kumamoto), Miike (Omuta, Fukuoka), Kuchinotsu (Minamishimabara, Nagasaki), and Oniike (Amakusa, Kumamoto). Five ferry routes cross the Ariake Sea.