The Mistassibi River is a river in central Quebec, Canada. It is 298 km long and has a drainage basin of 9325 km2. Its source is an area with unnamed lakes about 50 km east of Canso Bay of Lake Albanel, eastern neighbor of Lake Mistassini. The name Mistassibi, only in use since the late 19th century, comes from the Innu-aimun language, meaning "large river". Its alternate name used to be Rivière aux Foins (Hay River). Just like the nearby Mistassini River, the Mistassibi River runs in a north–south direction. It passes through Lac au Foin (Hay Lake) and among its tributaries are the aux Oiseaux, du Dépôt, and North-East Mistassibi Rivers. It drains into the Mistassini River at Dolbeau-Mistassini. The company Minashtuk Inc. operates a 12 MW run-of-the-river hydroelectric power station on the Mistassibi River since 2000.