Sarnia, city (1991 pop. 74,376), S Ont., Canada, on the St. Clair River, at the south end of Lake Huron and opposite Port Huron, Mich. The two cities are connected by a railroad tunnel, and there is a bridge between Port Huron and Point Edward, just N of Sarnia. The city is a port and handles a large volume of freight for transshipment from railroads to lake steamers. There are grain elevators, machinery plants, oil refineries, and chemical and synthetic-rubber industries.