The Rivière du Lac Onésime is a tributary of the Saguenay River, flowing in the city of Saguenay (city), in the administrative region of Saguenay–Lac-Saint-Jean, in the province of Quebec, in Canada.

Agriculture is the main economic activity in the Onésime lake river area; forestry, second.

The surface of the Lac Onésime river is usually frozen from the beginning of December to the end of March, however the safe circulation on the ice is generally made from mid-December to mid-March.

From its source (Onésime lake), the "rivière du Lac Onésime" flows north over 2.5 km (1.6 mi) with a drop of 102 m (335 ft) in forest and agricultural areas.

This confluence is located at: From the mouth of the Lac Onésime river, the current follows the course of the Saguenay River for 112 km (70 mi) east to Tadoussac where it merges with the Saint Lawrence estuary.