Demopolis is the largest city in Marengo County, Alabama, United States. The population was 7,483 at the time of the 2010 United States Census.The city lies within Webb's Bend at the confluence of the Black Warrior and Tombigbee rivers. It is situated atop a cliff composed of the Demopolis Chalk Formation, known locally as White Bluff, on the east bank of the Tombigbee River.