The Flapjack River is a tributary of the Mattawa Bay of the Southwest of Gouin Reservoir, flowing into the town of La Tuque, into the administrative area of the Mauricie, in Quebec, in Canada.

The surface of the Flapjack River is usually frozen from mid-November to the end of April, however, safe ice movement is generally from early December to late March.

The Flapjack River originates at the mouth of a Gut Lake (length: 0.8 kilometres (0.50 mi); altitude: 439 metres (1,440 ft)).

From there, the current flows over 164.9 kilometres (102.5 mi) until Gouin dam, according to the following segments: The term flapjack refers to a cake of British origin, resembling an energy bar.

The toponym "Flapjack River" was formalized on December 5, 1968 at the Commission de toponymie du Québec, when it was created.