city, seat of Lambton county, southeastern Ontario, east-central Canada, on the St. Clair River, at the southern end of Lake Huron, 55 miles (90 km) west of London. First visited by French explorers as early as 1627, its site was settled in 1807, and the present city was founded in 1833 by Malcolm Cameron. It was originally named Sarnia in 1836 from the Roman word for the English Channel island of Guernsey. Oil was discovered in the county in 1858, and by the turn of the 20th century the city had become a major petroleum centre.

Sarnia-Clearwater has complete harbour facilities, oil refineries (crude oil being supplied by pipeline from Alberta), extensive petrochemical industries, a salt industry, grain elevators, and a diversity of small manufactures. The Polymer Corporation plant, built in 1942, pioneered the manufacture of synthetic rubber in Canada. A bridge, railway tunnel, and ferry service connect Sarnia-Clearwater with Port Huron, Mich., on the west side of the river. Inc. town, 1856; city, 1914. Pop. (2006) 71,419.