Decatur is a city in Morgan and Limestone Counties in the U.S. state of Alabama. The city, affectionately known as "The River City", is located in Northern Alabama on the banks of Wheeler Lake, along the Tennessee River. It is the largest city and county seat of Morgan County. The population in 2010 census was 55,683.

Decatur is also the core city of the two-county large Decatur Metropolitan Area which had 150,125 in 2006. Combined with the Huntsville Metropolitan Area, the two create the Huntsville-Decatur Combined Statistical Area, of which Decatur is the second largest city.

Like many southern cities in the early 19th century, Decatur's early success was based upon its location along a river. Railroad routes and boating traffic pushed the city to the front of North Alabama's economic atmosphere. The city rapidly grew into a large economic center within the Tennessee Valley and was a hub for travelers and cargo between Nashville/Chattanooga and Mobile/New Orleans. Throughout the 20th century, the city experienced steady growth, but was eclipsed as the regional economic center by a fast growing Huntsville during the space race. The city now finds its economy heavily based on manufacturing industries, cargo transit, and hi-tech industries such as General Electric, and the United Launch Alliance.