The Saint-Athanase River is a tributary of the south shore of the Saguenay River flowing into the municipality of Petit-Saguenay in the Le Fjord-du-Saguenay Regional County Municipality, Quebec, Canada.

The surface of the Saint-Athanase River is usually frozen from the beginning of December to the end of March, however, safe ice circulation is generally from mid-December to mid-March.

The main hydrographic slopes near the Saint-Athanase River are: The Saint-Athanase River rises at the mouth of Petit Lac Alphée (length: 0.3 km (0.19 mi); altitude: 183 m (600 ft)).

This confluence is located at: The toponym "rivière Saint-Athanase" (St. Athanase River) refers to St. Athanasius, a patron of the Roman Catholic Church.

The toponym "Saint-Athanase River" was formalized on December 5, 1968, at the Bank of place names of the Commission de toponymie du Quebec.