| Saint-Cyr South | |
| Map: | Nottaway map.png |
| Subdivision Type1: | Country |
| Subdivision Name1: | Canada |
| Subdivision Type2: | Province |
| Subdivision Name2: | Quebec |
| Subdivision Type3: | Region |
| Subdivision Name3: | Nord-du-Québec |
| Length: | 50.1km (31.1miles)[1] |
| Source1: | Barry Lake (Saint-Cyr River South) |
| Source1 Location: | Eeyou Istchee James Bay (municipality), Nord-du-Québec, Quebec |
| Source1 Coordinates: | 48.9694°N -75.5428°W |
| Source1 Elevation: | 392m (1,286feet) |
| Mouth: | Mégiscane River |
| Mouth Location: | Eeyou Istchee James Bay (municipality), Nord-du-Québec, Quebec |
| Mouth Coordinates: | 48.5914°N -76.7711°W |
| Mouth Elevation: | 293m (961feet) |
| Tributaries Right: | Chartrand River |
The Saint-Cyr River South is a tributary of the Mégiscane River (via the Canusio Lake, flowing into Senneterre, in the administrative region of Abitibi-Témiscamingue, in Quebec, in Canada.
The river Saint-Cyr Sud flows successively into the cantons of Bailly, Kalm, Mesplet and Cherrier. Forestry is the main economic activity of the sector; recreational tourism activities, second.
The valley of the Saint-Cyr South River is served by the forest road R1053 (east–west direction) which passes on the north-west side and north of Lac Saint-Cyr. This road joins the road R1009 (north–south direction) which passes to the east of the Aigle River (Doda Lake).
The surface of the Saint-Cyr South River is usually frozen from early November to mid-May, however, safe ice circulation is generally from mid-November to mid-April.