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: 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: 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.