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 was 3,914 as of the 2020 Census, up 3.8% from the figure reported in the 2010 census of 3,770, which itself was an increase of 10.8 percent from the 3,410 reported 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.