Le Mars is a city and the county seat of Plymouth County, Iowa, United States. It is located on the Floyd River northeast of Sioux City. The p…Le Mars is a city and 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.