Mitsuishi River

 is a river in Hokkaido, Japan. It is in length and has a drainage area of .
Course.
The Mitsuishi River originates at Mount Setaushi () in the Hidaka Mountains and flows roughly south to southwest. The river flows into a reservoir at the Mitsuishi Dam. The Mitsuishi Dam was completed on the river in 1992 to prevent the flow of earth and sand from the , which is prone to landslides. It then continues until it reaches the Pacific Ocean at Mitsuishi in Shinhidaka, Hokkaido.