Kosai is a city located in far western Shizuoka Prefecture, Japan. As of 30 June 2019, the city had an estimated population of 57,885 in 24,232 households, and a population density of 668.7 persons per km2. The total area of the city is 85.65 square kilometres (33.07 sq mi).

Shizuoka Prefecture is a prefecture of Japan located in the Chūbu region of Honshu. As of September 2023, Shizuoka Prefecture has a population of 3,555,818 and has a geographic area of 7,777.42 km2 (3,002.88 sq mi). Shizuoka Prefecture borders Kanagawa Prefecture to the east, Yamanashi Prefecture to the northeast, Nagano Prefecture to the north, and Aichi Prefecture to the west.