The rivière du Grand Portage Sud-Ouest (English: Grand Portage South-West River) is a tributary of rivière du Grand Portage, flowing into the unorganized territory of Lac-Ashuapmushuan in the Le Domaine-du-Roy Regional County Municipality, in the administrative region of Saguenay–Lac-Saint-Jean, in the province of Quebec, in Canada.

[2] The Grand Portage Sud-Ouest river draws its source from Lake Hary (length: 0.7 km (0.43 mi); altitude: 569 m (1,867 ft)).

This spring is located in a mountainous area in the unorganized territory of Lac-Ashuapmushuan, at: From the mouth of Lake Hary, the Grand Portage Sud-Ouest river flows over 17.7 km (11.0 mi) with a drop of 139 m (456 ft), entirely in forest areas, according to the following segments: The Rivière du Grande Portage Sud-Ouest flows onto the south bank of the Rivière du Grand Portage.

This confluence is located at: From the mouth of the Grand Portage Sud-Ouest river, the current successively descends the course of the Grand Portage river on 24.8 km (15.4 mi), the course of the Chigoubiche river on 46.2 km (28.7 mi), the course of the Ashuapmushuan River over 59.8 km (37.2 mi), then cross lac Saint-Jean eastward on 41.1 km (25.5 mi) (i.e. its full length), follows the course of the Saguenay River via la Petite Décharge on 172.3 km (107.1 mi) east to Tadoussac where it meets the estuary of Saint Lawrence.

[2] The toponym "rivière du Grand Portage Sud-Ouest" was made official on December 5, 1968, at the Place Names Bank of the Commission de toponymie du Québec.