Ottawa (river, Canada) (French Rivière des Outaouais), river in southeastern Canada, a major tributary of the St. Lawrence River, 1,120 km (696 mi) long. It rises in southwestern part of the province of Québec, in a region of the Canadian Shield, and flows west through a series of lakes until it forms Lake Timiskaming. It then flows southeast for about 640 km (about 400 mi) to the St. Lawrence River, near Montréal; in most of this section of the river the Québec-Ontario provincial border is located in midstream.

The principal tributaries of the Ottawa are the Coulonge, Gatineau, and Lièvre rivers from the north, and the Madawaska and Rideau rivers from the south. Cities on the Ottawa include Ottawa (the capital of Canada) and Pembroke in Ontario; and Hull, in Québec. The first European to visit the river was probably the French explorer Étienne Brûlé, in 1610. The Ottawa was an important early transportation route into the western interior, and was controlled by the Algonquin tribe. It was the primary route for the fur trade until the early 19th century, when the Rideau Canal linking Ottawa and Lake Ontario was built. Lumbering then became the river valley's chief industry for the next hundred years, an industry now in decline and marked by high unemployment. Hydroelectric development came to the Ottawa relatively late because of political disputes, but the river now supplies a large percentage of the power for Québec and Ontario.