The Muy River is a tributary of the east bank of the Wetetnagami River flowing into the Regional County Municipality (RCM) of Eeyou Istchee James Bay, in the administrative region of Nord-du-Québec, Quebec, Canada.

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

The lower part of the Muy River flows through the Lake Wetetnagami Biodiversity Reserve.

The confluence of the Muy River with the Wetetnagami Lake is located at: At various times in history, this territory has been occupied by the Attikameks, the Algonquin and the Cree.

The toponym "rivière Muy" was officialized on December 5, 1968, at the Commission de toponymie du Québec, when it was created.