Le Mars is the county seat of Plymouth County, Iowa, United States. It is located on the Floyd River northeast of Sioux City. The population was 10,571 at the time of the 2020 census. Le Mars is part of the Sioux City metropolitan area.

Plymouth County is a county located in the U.S. state of Iowa. As of the 2020 census, the population was 25,698. The county seat is Le Mars. Plymouth County was named after Plymouth, Massachusetts.