Red Deer River has a total length of 724 km and a drainage area of 45,100 km². Its mean discharge is 70 m³/s.
The river got its name form the translation of Was-ka-soo which means "elk river" in the Cree.
The river originates on the eastern slopes of the Canadian Rockies, in Banff National Park, and then flows east through the mountains and foothills region. It turns north-east before Sundre and keeps this heading to the city of Red Deer, where it turns east, and then south before Stettler. It flows south with its valley protected by provincial and regional parks such as Tolman Badlands Heritage Rangeland, Dry Island Buffalo Jump Provincial Park, Dry Island Corridor and Midland Provincial Park. At Drumheller it has a south-east direction, and while it flows through Dinosaur Provincial Park it turns east and flows to the Alberta/Saskatchewan border, which it crosses at Empress. It flows for 16 km through Saskatchewan before it merges into the South Saskatchewan River.
The waters of Ewing Lake, Little Fish Lake also flow into the Red Deer River.