The Mazarin River is a tributary of the Ashuapmushuan River, flowing into the unorganized territory of Lac-Ashuapmushuan, Quebec, into the Regional County Municipality (MRC) of Le Domaine-du-Roy, in the administrative region of Saguenay-Lac-Saint-Jean, in Quebec, in Canada.

The Mazarin river flows entirely in the canton of Théberge.

The surface of the Mazarin River is usually frozen from early November to mid-May, however, safe ice circulation is generally from mid-November to mid-April.

The Mazarin River originates at the mouth of lake Mazarin (length: 2.8 kilometres (1.7 mi) altitude: 327 metres (1,073 ft)).

The mouth of this head lake is located at: From the mouth of the Mazarin Lake, the course of the Mazarin River flows on 15.1 kilometres (9.4 mi) according to the following segments: The confluence of the Mazarin River with the Normandin River is located at: The toponym "Rivière Mazarin" was officialized on December 5, 1968, at the Commission de toponymie du Québec, when it was created.