Algonac is a city in St. Clair County of the U.S. state of Michigan. The population was 4,196 at the 2020 census. Incorporated as a village in 1867 and again as a city in 1967, Algonac is located at the southern end of the St. Clair River and contains a long boardwalk and riverfront park. Algonac State Park is located just north of the city. The city is also notable for the founding and headquarters of the now-defunct Chris-Craft Boats company.