Decatur is a city in Root and Washington townships, Adams County, Indiana, United States. It is the county seat of Adams County. Decatu…Decatur is a city in Root and Washington townships, Adams County, Indiana, United States. It is the county seat of Adams County. Decatur is home to Adams Memorial Hospital, which was designated as one of the "Top 100" Critical Access Hospitals in the United States. The population of Decatur was 9,913 at the 2020 census, up from 9,405 at the 2010 census.