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. The site's natural harbour first attracted the French explorer La Salle.