Decatur is a city in, and the county seat of, DeKalb County, Georgia, United States, part of the Atlanta metropolitan area. With a populatio…Decatur is a city in, and the county seat of, DeKalb County, Georgia, United States, part of the Atlanta metropolitan area. With a population of 24,928 in the 2020 census, the municipality is sometimes assumed to be larger since multiple ZIP Codes in unincorporated DeKalb County bear Decatur as the address.