Kosai (湖西市, Kosai-shi) is a city located in far western Shizuoka Prefecture, Japan.

Due to its location, the warm Kuroshio Current offshore provides for a temperate maritime climate with hot, humid summers and mild, cool winters.

During the establishment of the modern municipalities system of the early Meiji period in 1889, the area was reorganized into five villages within Hamana District, Shizuoka.

Four of these villages (Washizu, Iride, Shirasuka, Chibata) merged to form the city of Kosai on January 1, 1972.

[5] Kosai has a mayor-council form of government with a directly elected mayor and a unicameral city legislature of 18 members.

The economy of Kosai is dominated by light manufacturing of automobile components and electronics, as well as commercial fishing.