Lake St. Clair is not one of the Great Lakes but it separates Lake Erie and Lake Huron, along with the Detroit River and the St. Clair River. It lies between Ontario and Michigan and is about 10 km (6 mi) northeast of Detroit and Windsor, Ontario.

Lake St. Clair is 42 km (26 mi) from north to south and 37 km (24 mi) from east to west. Its totals surface area is 1114 km² (430 mi²). It is a very shallow lake with an average depth of about 10 feet (3 metres), and a maximum natural depth of 21 feet (6.4 metres), although it is 27 feet (8.2 metres) deep in the navigation channel which has been dredged for freighter passage. At the northeastern end of Lake St. Clair is the extensive delta system of the St. Clair River -- the largest freshwater delta in the world. The Thames River flows westward through southwestern Ontario into the lake.

In 1679 French explorers named it Lac Sainte Claire as they discovered it on the feast day of Sainte Claire of Assisi.