The Rivière du Sud-Ouest is a tributary of the southeast coast of the St. Lawrence River. This river flows in the administrative region of Bas-Saint-Laurent, in Quebec, in Canada, in the regional county municipalities of:

• Les Basques Regional County Municipality: municipality of Saint-Mathieu-de-Rioux;
• Rimouski-Neigette Regional County Municipality: city of Rimouski (sector Le Bic).

The Southwest river rises in the outlet of Lake Saint-Mathieu (length: 5.3 metres (17 ft); altitude: 114 metres (374 ft)), at Saint-Mathieu-de-Rioux. It then follows the St. Lawrence River in a northeast-southwest direction to flow into the Havre du Bic. During its course, it crosses Petit Lac Saint-Mathieu and Lac de la Station. Its two main tributaries are the outlet of Grand Lac Malobès and the Neigette River. A fall from a height of 18 metres (59 ft) is located near its mouth.

The flow of the south-west river is generally slow, its course being located on a plateau whose average altitude is 105 kilometres (65 mi). The last eight kilometers show more rugged terrain.

From the mouth of Lake Saint-Mathieu, the southwest river flows over 7.1 kilometres (4.4 mi) divided into the following segments:

Upper course of the river (segment of 12.1 kilometres (7.5 mi))

• 0.2 kilometres (0.12 mi) towards the northeast, crossing under the bridge of the Ladrière road, to the southwest shore of Petit lac Saint-Mathieu;
• 2.9 kilometres (1.8 mi) towards the north-east, crossing the full length of Petit lac Saint-Mathieu;
• 2.3 kilometres (1.4 mi) towards the northeast, crossing the road to 5th rang, collecting the waters of the Neigette River (Les Basques), up to the route du 3th Rang Est;
• 1.9 kilometres (1.2 mi) northwards, to the limit of Saint-Simon-de-Rimouski;
• 2.6 kilometres (1.6 mi) towards the northeast, constituting the limit between Saint-Mathieu-de-Rioux and Saint-Simon-de-Rimouski;
• 0.7 kilometres (0.43 mi) north-east, in Saint-Simon-de-Rimouski, to the road bridge of 2th rang Ouest;
• 1.5 kilometres (0.93 mi) towards the northeast, forming the limit between Saint-Fabien and the town of Rimouski (sector Le Bic);

Upper course of the river (segment of 19.4 kilometres (12.1 mi))

• 7.4 kilometres (4.6 mi) towards the northeast, in Rimouski, recovering the waters of the outlet of Lac des Coulombe (coming from the north), crossing Lac de la Station (length: 2.7 kilometres (1.7 mi); altitude: 106 metres (348 ft)) over its full length, to the dam located to the northeast;
• 0.6 kilometres (0.37 mi) eastwards, to the bridge of 7th Avenue which crosses the village of Saint-Fabien;
• 2.5 kilometres (1.6 mi) north-east, up to the confluence of the outlet of Grand Lac Malobès;
• 5.1 kilometres (3.2 mi) north-east, passing under the Canadian National railway at the start of this segment, to the route 132 bridge;
• 3.8 kilometres (2.4 mi) towards the north-east, crossing the Bay of Roses and passing under the bridge of Chemin du Petit Portage, to its confluence.

The watershed has an area of 192 square kilometres (74 sq mi).

The toponym "south-west river" was formalized on December 5, 1968 at the Commission de toponymie du Québec.

The river hosts a small population of Atlantic salmon of around 40 spawners. The latter goes up the river on 4 square kilometres (1.5 sq mi) where the fall of 18 metres (59 ft) constitutes an insurmountable obstacle. Salmon spend the first two years of their lives in the river. The second species in abundance is generally the American eel, which unlike salmon, goes up the river over its entire length to Lake Saint-Mathieu.

• Union québécoise pour la conservation de la nature (2004). Synthesis report: Parc national du Bic (PDF). p. 91.

• Saint-Mathieu-de-Rioux, a municipality
• Saint-Fabien, a municipality
• Rimouski, a city
• Neigette River, a stream
• Bic National Park
• Rimouski-Neigette Regional County Municipality
• Les Basques Regional County Municipality
• St. Lawrence River