The rivière du Grand Portage Sud-Est (English: Grand Portage South-East 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.

The lower part is served indirectly by forest road R0204 which connects north to route 167.

[2] The Grand Portage Sud-Est river draws its source from Lac Demers (length: 2.2 km (1.4 mi); altitude: 497 m (1,631 ft)).

This spring is located in a mountainous area in the unorganized territory of Lac-Ashuapmushuan, at: From the mouth of Lac Demers, the Grand Portage Sud-Est river flows over 6.8 km (4.2 mi), with a drop of 117 m (384 ft), entirely in forest area, according to the following segments: The Rivière du Grand Porcharge Sud-Est flows onto the south bank of the Rivière du Grand Portage.

This confluence is located at: From the mouth of the Rivière du Grand Portage Sud-Est, the current successively descends the course of the rivière du Grand Portage onto 18.0 km (11.2 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) eastward to Tadoussac where it merges with the estuary of Saint Lawrence.