is a city in Root and Washington townships, Adams County, Indiana, United States. The population was 9,528 at the 2000 census. The city is the county seat of Adams County. This town was named after the prominent war hero of the time Stephen Decatur, Jr.. He was one of the captains of the original 6 frigates of the US navy. When Indiana officially became a state in 1816, Decatur's name was as well known and revered then as any celebrity today. The town was founded in 1836.
According to the United States Census Bureau, the city has a total area of 4.9 square miles (12.8 km²), of which, 4.9 square miles (12.7 km²) of it is land and 0.20% is water.