Isahaya, city, southern Nagasaki ken (prefecture), western Kyushu, Japan. It is located on an isthmuslike strip of land at the juncture of Shimabara (east) and Nagasaki (west) peninsulas and the Tara Volcano (Tara-dake) massif (north). The western part of the city lies at the southeastern extremity of Ōmura Bay, the Ariake Sea is just to the east, and southern Isahaya extends to the northern shore of Tachibana Bay.
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 Edo (Tokugawa) period (1603–1867). Since the late 19th century, coastal reclamation along the Ariake Sea has increased the area used for the cultivation of high-quality rice. Isahaya is now a local administrative and cultural centre, with good road and rail connections to Nagasaki, about 10 miles (16 km) to the southwest. Its economy is based on the food-processing industry and on tourism to Unzen-Amakusa National Park and Tara-dake Prefectural Park. Pop. (2010) 140,752; (2015) 138,078.