The Gentilly South-West River (in French: rivière Gentilly Sud-Ouest) is a tributary of the Gentilly River which flows on the south shore of the St. Lawrence River.

The Gentilly Sud-Ouest river flows in the territory of the municipality of Saint-Sylvère and the town of Bécancour, in the MRC of Bécancour Regional County Municipality, in the administrative region of Centre-du-Québec, in Québec, in Canada.

The main neighboring hydrographic slopes of the Gentilly Sud-Ouest river are:

• north side: Gentilly River, St. Lawrence River;
• east side: Gentilly River;
• south side: Poulet stream, Bécancour River;
• west side: Grand-Saint-Louis River, Bécancour River.

The Gentilly Sud-Ouest river draws its source in an agricultural zone, from the confluence of two streams, in the eighth rang of Saint-Sylvère. This area is located north of the Bécancour River, 7 kilometres (4.3 mi) southwest of the village of Saint-Sylvère and northeast of the village of Saint-Wenceslas.

From its head area, the Gentilly Sud-Ouest River flows with a drop of 70 metres (230 ft) over 25.0 kilometres (15.5 mi) in the following segments:

• 1.0 kilometre (0.62 mi) to the southwest in an agricultural zone;
• 2.7 kilometres (1.7 mi) north, to the sixth rang road;
• 2.1 kilometres (1.3 mi) north, to the limit between Saint-Sylvère and Bécancour;
• 2.3 kilometres (1.4 mi) north, to Chemin des Cerisiers;
• 2.1 kilometres (1.3 mi) north to Chemin des Hêtres;
• 1.7 kilometres (1.1 mi) north, to the road;
• 6.0 kilometres (3.7 mi) north, winding up to Route des Ormes;
• 3.0 kilometres (1.9 mi) north, winding up to Boulevard du Parc-Industriel;
• 4.1 kilometres (2.5 mi) (or 2.6 kilometres (1.6 mi) in a direct line) north, winding up to its mouth.

The Sud-Ouest Gentilly River flows onto the west bank of the Gentilly River, in Bécancour.

The toponym “rivière Gentilly Sud-Ouest” was made official on December 5, 1968, at the Commission de toponymie du Québec.

• List of rivers of Quebec