The Assomption River (in French Rivière l'Assomption, named after the Assumption of Mary) is the most important waterway in the Lanaudière region of Quebec, Canada. It is over 200 km (120 mi) long. It has a drainage basin of 4,220 km2 (1,630 sq mi). It flows from the Mont Tremblant massif. Seven significant rivers flow into the Assomption (de la Boule, Versailles, Noire, la Chaloupe, Ouareau, Saint-Esprit, and Achigan) before it flows into the Rivière des Prairies at Repentigny, Quebec.
About 150000 people live in the drainage area of this river.