The Assomption River (in French Rivière l'Assomption [ʁivjɛʁ lasɔ̃psjɔ̃], named after the Assumption of Mary) is the most important waterway in the Lanaudière region of Quebec, Canada.

And there are even mountains of over 600 metres (2,000 ft) in the Mont Tremblant massif where the L'Assomption River forms.

The steeper slopes of the highlands create a fast current whereas the lowland flats produces a slower flow.

The L'Assomption River watershed has 490 lakes of which 24 have a surface area larger than 1 square kilometre (250 acres).

Over 150,000 people live in the river's watershed in 43 municipalities, of which the most important are Repentigny, Joliette, Le Gardeur, and L'Assomption.

Agriculture has a dominant position along the river: in 1995 there were 1,305 producers with 60,221 hectares (148,810 acres) under cultivation and possessing 73,563 animals.