Isahaya, World Photo/Shostal Associates city, Nagasaki ken (prefecture), Kyushu, Japan, at the neck of the three peninsulas of Shimabara, Nagasaki, and Nishisonoki. The city has been an important commercial centre for the surrounding agricultural area since the 13th century. It became a castle town of the Isahaya family in the Tokugawa era (1603–1867). Since the late 19th century, coastal reclamation has increased the area used for the cultivation of high-quality rice. Isahaya is now a local administrative and cultural centre. Its economy is based on the food-processing industry and tourism to Unzen-Amakusa National Park and Tara-dake Prefectural Park. Pop. (2005) 144,034.