Isahaya (諫早市, Isahaya-shi) is a city located in Nagasaki Prefecture, Japan. The city was founded on September 1, 1940.
As of March 2008, the city has an estimated population of 141,151 and a population density of 439 persons per km². The total area is 320.99 km².
On March 1, 2005, the towns of Tarami, Moriyama, Iimori, Takaki and Konagai (all from Kitatakaki District) were merged to create the new and expanded city of Isahaya. The city was once part of Kitatakaki District, but due to this merger, the district has no more municipalities left and the district was officially dissolved as a result of this merger.