The lac Castelnau is a freshwater body of the watershed of the rivière à la Chasse, in the territory of the town of Baie-Comeau, in the Manicouagan Regional County Municipality on the administrative region of Côte-Nord, in the province of Quebec, to Canada.

[2] The surface of Lake Castelnau is usually frozen from the beginning of December to the end of March; however, traffic on the ice is generally safe from mid-December to mid-March.

Lake Castelnau is located in the northern part of the territory of the town of Baie-Comeau.

[2] From the mouth of Lake Castelnau, the current descends on 9.9 km (6.2 mi) generally north-east, then south, following the course of the rivière à la Chasse, in particular by crossing an urban sector in the eastern part of Baie-Comeau at the end of the segment, to flow onto the north shore of the Manicouagan estuary.

[3] The toponym "lac Castelnau" was formalized on December 5, 1968, at the Place Names Bank of the Commission de toponymie du Québec.