Marysville is a city in St. Clair County of the U.S. state of Michigan. The population was 9,959 at the 2010 census. The municipality was founded in 1919, first as a village, then became a city in 1924.

St. Clair County is a county located in the U.S. state of Michigan and bordering the west bank of the St. Clair River. As of the 2020 census, the population was 160,383. It is the 13th-most populous county in the state. The county seat is Port Huron, located at the north end of the St. Clair River at Lake Huron. The county was created September 10, 1820, and its government was organized in 1821. It is located northeast of Detroit and is included within the Detroit–Warren–Dearborn Metropolitan Statistical Area by the Office of Management and Budget. It is also considered by the State of Michigan to be a part of The Thumb, a peninsula that is surrounded by Lake Huron in the east-central area of the state. This area is sometimes dubbed the Blue Water Area.