Kōshi is a city in Kumamoto Prefecture, Japan. As of December 2013, the city has an estimated population of 58,142 and a population density of 1093.51 persons per km². The total area is 53.17 km². The modern city of Kōshi was established on February 27, 2006, from the merger of former town of Kōshi, with the town of Nishigoshi (both from Kikuchi District). It was the 14th "Great Heisei Merger" in Kumamoto Prefecture.