Sebastian County is a county located in the U.S. state of Arkansas. As of the 2020 census, the population was 127,799, making it the fourth-most populous county in Arkansas. The county has two county seats, Greenwood and Fort Smith.

A county seat is an administrative center, seat of government, or capital city of a county or civil parish. The term is in use in five countries: Canada, China, Hungary, Romania, and the United States. An equivalent term, shire town, is used in the U.S. state of Vermont and in several other English-speaking jurisdictions. County towns usually serve a similar function in Ireland, Jamaica, and the United Kingdom.