Lamar County is a county located in the west central portion of the U.S. state of Georgia. As of the 2020 census, the population was 18,500. The county seat is Barnesville.

Barnesville is a city in Lamar County, Georgia, United States. As of the 2010 census, the city had a population of 6,755, up from 5,972 at the 2000 census. The city is the county seat of Lamar County and is part of the Atlanta metropolitan area.