Kai Province (甲斐国, Kai no kuni^?) is an old province in Japan that corresponds to Yamanashi Prefecture today. It lies in central Honshū, west of Tokyo, in a landlocked mountainous region that includes Mount Fuji along its border with Shizuoka Prefecture. Another name for the province is Kōshū (甲州).

During the Sengoku period, the warlord Takeda Shingen ruled Kai from his stronghold at Kōfu.

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