St. Clair is a township in southwestern Ontario, Canada, immediately south of Sarnia in Lambton County, along the eastern shores of the St. Clair River.

Lambton County is a county in Southwestern Ontario, Canada. It is bordered on the north by Lake Huron, which is drained by the St. Clair River, the county's western border and part of the Canada-United States border. To the south is Lake Saint Clair and Chatham-Kent. Lambton County's northeastern border follows the Ausable River and Parkhill Creek north until it reaches Lake Huron at the beach community of Grand Bend. The county seat is in the Town of Plympton-Wyoming.