Ottawa Valley is the valley along the boundary between Eastern Ontario and Western Quebec along the Ottawa River. The valley is the transition between the Saint Lawrence Lowlands and the Canadian Shield. Because of the surrounding shield, the valley is narrow at its western end, then becomes increasingly wide (mainly on the Southern Ontario side of the river) as it progresses eastward. The underlying geophysical structure is the Ottawa-Bonnechere Graben. Approximately 1.3 million people reside in the valley (and along its tributaries), of these the majority, around 80%, reside in Ottawa, the remainder on the north side of the Ottawa River, in Quebec. The total area of the Ottawa Valley is 2.4 million hectares (6 million acres). The National Capital Region area has just over 1.4 million inhabitants in both provinces.

Near the city of Ottawa, the Ottawa Valley merges with the St. Lawrence Valley to the south to create a delta of flat farmland stretching unbroken from the Ottawa River to the Saint Lawrence River as far east as the island of Montreal, where the two rivers meet. This area is sometimes referred to as the "Lower Ottawa Valley", in contrast with the "Upper Ottawa Valley" west of Ottawa, but the name is not common, and most people think of the Ottawa Valley as only the upper portion.

From west to east, the major Ottawa Valley communities are Mattawa, Deep River (with nearby Chalk River, the site of Canada's nuclear reactor program), Petawawa (a major Canadian military base), Pembroke (where Samuel de Champlain landed briefly), Fort Coulonge, Shawville, Renfrew, Quyon, Arnprior, Ottawa (the nation's capital), Rockland, L'Orignal, Hawkesbury, and Rigaud.