Smith County is a county in the U.S. state of Tennessee. As of the 2020 census, the population was 19,904. Smith County is located in the region of the state known as Middle Tennessee. Its county seat is Carthage. The county was organized in 1799 and is named for Daniel Smith, a Revolutionary War veteran who made the first map of Tennessee and served as a United States senator. Smith County is part of the Nashville-Davidson–Murfreesboro–Franklin, TN Metropolitan Statistical Area.