The Assomption River is the most important waterway in the Lanaudière region of Quebec, Canada. It is over 200 kilometres (120 mi) long, and has a drainage basin (watershed) of 4,220 square kilometres (1,630 sq mi). Its source is the Mont Tremblant massif. Seven significant rivers flow into the Assomption before it flows into the Rivière des Prairies at Repentigny, Quebec.

Repentigny is an off-island suburb of Montreal, Quebec, Canada. It is located north of the city on the lower end of the L'Assomption River, and on the Saint Lawrence River. Repentigny and Charlemagne were the first towns off the eastern tip of the Island of Montreal. Repentigny is part of the Lanaudière region.