Decatur is a city in Decatur County, Iowa, United States. The population was 175 at the time of the 2020 census. Decatur was the first county …Decatur is a city in Decatur County, Iowa, United States. The population was 175 at the time of the 2020 census. Decatur was the first county seat of Decatur County, until 1853, when the seat was transferred to Leon.