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

During the Sengoku Period, the warlord Takeda Shingen ruled Kai from his stronghold at Kofu.