Rivière-Mistassini is an unorganized territory in the Canadian province of Quebec, located in the regional county municipality of Maria-Chapdelaine. The territory had a population of 27 as of the Canada 2021 Census, and covered a land area of 17,629.74 km2. The eponymous Mistassini River has its source in the far northern part of the territory and entirely bisects it from north to south.