Sumter is a city in and the county seat of Sumter County, South Carolina, United States. Known as the Sumter Metropolitan Statistical Area, the namesake county adjoins Clarendon and Lee to form the core of Sumter-Lee-Clarendon Tri-county area of South Carolina that includes three counties straddling the border of the Sandhills, Pee Dee, and Lowcountry regions. The population was 39,643 at the 2000 census, and it rose to 40,524 at the 2010 census.