Marshall County is a county located on the south central border of Oklahoma. As of the 2020 census, the population was 15,312. Its county seat is Madill. The county was created at statehood in 1907 from the former Pickens County of the Chickasaw Nation. It was named to honor the maiden name of the mother of George Henshaw, a member of the 1906 Oklahoma Constitutional Convention. The county and its cities are part of the Texoma region.

Madill is a city in and the county seat of Marshall County, Oklahoma, United States. It was named in honor of George Alexander Madill, an attorney for the St. Louis-San Francisco Railway. The population at the 2010 census was 3,770, an increase of 10.8 percent from 3,410 at the 2000 census. It is best known as the site of the annual National Sand Bass Festival. It is part of the Texoma region.