The Saint-Cyr River /ˌseɪntˈsɪər/ is a tributary of the eastern shore of the Ottawa River.

The Saint-Cyr River flows into the Municipality Rapides-des-Joachims, then from Sheenboro into the Regional County Municipality (MRC) Pontiac Regional County Municipality, in Outaouais administrative region, in Quebec, in Canada.

"Lac la Truite" (English: Troot Lake) (length: 2.4 km, altitude: 355 m) is the headwaters of the Saint-Cyr River.

From "Lac la Truite", the Saint-Cyr River flows over: The former name of the river was "Boom West Brook", as opposed to the branch being referred to as "Boom Creek" in this hydrographic slope.

The toponym "Saint-Cyr River" was formalized on December 5, 1968, at the Commission de toponymie du Québec.