Decatur () is the largest city and county seat of Morgan County (with a portion also in Limestone County) in the U.S. state of Alabama. Nicknamed "The River City," it is located in northern Alabama on the banks of Wheeler Lake along the Tennessee River. The population was 57,938 at the 2020 census.

Decatur is the core city of the two-county large Decatur metropolitan area, with an estimated population of 157,425 in 2022. 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 and Mobile, as well as Chattanooga and New Orleans. Throughout the 20th century, the city experienced steady growth but was eclipsed as the regional economic center by the fast-growing Huntsville during the space race. Decatur now finds its economy heavily based on manufacturing, mining, cargo transit, chemical, and high-tech companies such as Vulcan Materials, Daikin, Toray, and United Launch Alliance.