Marshall County is a county located in the U.S. state of Iowa. As of the 2020 census, the population was 40,105. The county seat and largest city is Marshalltown. The county was formed on January 13, 1846, and named after John Marshall, Chief Justice of the United States Supreme Court.

Iowa is a landlocked state in the upper Midwestern region of the United States. It borders the Mississippi River to the east and the Missouri River and Big Sioux River to the west; Wisconsin to the northeast, Illinois to the east and southeast, Missouri to the south, Nebraska to the west, South Dakota to the northwest, and Minnesota to the north.