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.

In July 1957, a severe downpour within Nagasaki caused a large flood in Isahaya, with over 500 casualties and 3500 injuries reported in Isahaya alone.

Located in the central part of Nagasaki Prefecture. To the north of Isahaya City are mountainous lands of the Takayama mountain range. To the west is the base of the Nagasaki Peninsula and to the south is the base of Shimabara Peninsula. The northeastern part of the city area is surrounded by the three seas of Omura Bay in the northwest, Ariake Sea (Isahaya Bay) on the east side and Tachibana Bay on the south side.

Major land reclamation projects have occurred which added plains in the Ariake Sea. The biggest is the Isahaya Bay reclamation project (諫早湾干拓事業) by the Ministry of Agriculture, Forestry and Fisheries that started in 1989. Embankments and sluice gates were built and a total of 35 km2 has been reclaimed as of 2018.

Surrounding municipalities

• Nagasaki Prefecture
  • Nagasaki
  • Ōmura
  • Unzen
  • Nagayo
• Saga Prefecture
  • Tara