Marine City is a city in St. Clair County in the U.S. state of Michigan. Located on the west bank of the St. Clair River, it is one of the cities in the River District north of Detroit and south of Lake Huron. In the late 19th century, it was a major center of wooden shipbuilding and lumber processing. The population was 4,079 at the 2020 census. The city formerly featured an international auto ferry service, The Bluewater Ferry to Sombra, Ontario, Canada across the river.