Koumi is a town located in Nagano Prefecture, Japan. As of 1 April 2019, the town had an estimated population of 4,676 in 2012 households, and a population density of 41 persons per km2. The total area of the town is 114.20 square kilometres (44.09 sq mi).

Nagano Prefecture is a landlocked prefecture of Japan located in the Chūbu region of Honshū. Nagano Prefecture has a population of 2,007,682 and has a geographic area of 13,561.56 square kilometres (5,236.15 sq mi). Nagano Prefecture borders Niigata Prefecture to the north, Gunma Prefecture to the northeast, Saitama Prefecture to the east, Yamanashi Prefecture to the southeast, Shizuoka Prefecture and Aichi Prefecture to the south, and Gifu Prefecture and Toyama Prefecture to the west.