Cornwall (2001 population 45,640, metropolitan population 57,581) is a city in southeastern Ontario, Canada, located on the St. Lawrence River near the Quebec border. It is approximately 100 kilometres from Ottawa, Canada's capital, and Montreal, Quebec's largest metropolis.

The city was first settled in 1784 by United Empire Loyalists escaping the American Revolution from New York. First named New Johnstown, Cornwall was renamed for Prince George, the Duke of Cornwall.

Cornwall is a port-of-entry into Canada, and is connected to Massena, New York, USA via the Seaway International Bridge.

Several manufacturers are currently located in Cornwall, the most prominent of which being Domtar, a Quebec-based company running a paper mill employing nearly 1000 Cornwallites.

West of Cornwall, several small communities, known as The Lost Villages, were permanently flooded in 1958 by the construction of the St. Lawrence Seaway.