Public beach on Lake St. Clair in St. Clair Shores, Michigan

Lake St. Clair (French: Lac Sainte-Claire) is a lake between Ontario, Canada and Michigan in the United States, located about 10 km (6 mi) northeast of Detroit and Windsor, Ontario. Approximately 1114 km² (430 mi²) in area, the lake is part of the Great Lakes system, although it is not considered to be one of the Great Lakes. The lake, along the St. Clair River and Detroit River, provides the connection between Lake Huron to the north and Lake Erie to the south.

The lake is 42 km (26 mi) from north to south and 37 km (24 mi) from east to west. 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. The lake is fed from Lake Huron at its by the St. Clair River, which has an the extensive delta, the largest within the Great Lakes system. The Thames River also enters the lake from the east in southwestern Ontario. The lake is drained on its southwest end into Lake Erie by the Detroit River.

The lake served as part of the extensive navigational system of the Great Lakes for First Nations/Native Americans. On August 12, 1679 an expedition led by René Robert Cavelier, Sieur de La Salle named it Lac Sainte-Claire as they discovered it on the feast day of Saint Clare of Assisi.

On the Michigan side, most of the lake shore is lined by the wealthier northern suburbs of Detroit, known as the Grosse Pointe communities. Public access to the lake is highly restricted in this area, limited to private marinas and parks that are open only to residents of the specific community. Several sailing clubs are located along this shore, including the Grosse Pointe Yacht Club in Grosse Pointe Shores.