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.

Ōita Prefecture is a prefecture of Japan located on the island of Kyūshū. Ōita Prefecture has a population of 1,136,245 and has a geographic area of 6,340 km2. Ōita Prefecture borders Fukuoka Prefecture to the northwest, Kumamoto Prefecture to the southwest, and Miyazaki Prefecture to the south.