Ōtsuki is a city located in Yamanashi Prefecture, Japan. As of 1 October 2023, the city had an estimated population of 21,835 in 10207 households, and a population density of 86 persons per km2. The total area of the city is 380.25 square kilometres (146.82 sq mi).

Yamanashi Prefecture is a prefecture of Japan located in the Chūbu region of Honshu. Yamanashi Prefecture has a population of 817,192 and has a geographic area of 4,465 km2. Yamanashi Prefecture borders Saitama Prefecture to the northeast, Nagano Prefecture to the northwest, Shizuoka Prefecture to the southwest, Kanagawa Prefecture to the southeast, and Tokyo to the east.