Sarnia is a city in Lambton County, Ontario, Canada. It had a 2021 population of 72,047, and is the largest city on Lake Huron. Sarnia is located on the eastern bank of the junction between the Upper and Lower Great Lakes, where Lake Huron flows into the St. Clair River in the Southwestern Ontario region, which forms the Canada–United States border, directly across from Port Huron, Michigan.

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.