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.^[1]

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.^[2]

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.^[3]

The toponym "south-west river" was formalized on December 5, 1968 at the Commission de toponymie du Québec.^[4]

The river hosts a small population of Atlantic salmon of around 40 spawners.^[5] 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.^[6] 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.^[7]