Madill is a city and 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.