Nakatsu is a city on the northern border of Ōita Prefecture in Kyushu, Japan. As of 31 October 2023, the city had an estimated population of 82,301 in 41222 households, and a population density of 96 persons per km2. The total area of the city is 491.53 km2 (189.78 sq mi).

The Yamakuni River is a river that flows through the Ōita Prefecture in Japan. In its lower reaches it provides the boundary to the Fukuoka Prefecture to the west.