The Yahagi River (Yahagi-gawa) is a river that flows from Nagano Prefecture's Mount Ōkawairi, through Gifu Prefecture, and enters Mikawa Bay from Aichi Prefecture in Japan. It is one of Japan's first-class rivers.
After flowing south from Mount Ōkawairi, the river enters Gifu Prefecture and forms the prefectural border with Aichi Prefecture between the cities of Ena and Toyota. Downstream, the Yahagifuru River (矢作古川 Yahagifuru-kawa) follows the original path of the river; however, the river's path was changed to its current one at the beginning of the Edo period because of flooding. The river forms the border between the cities of Nishio and Hekinan when it flows into Mikawa Bay.