Antietam Creek (/ænˈtiːtəm/) is a 41.7-mile-long (67.1 km)[1] tributary of the Potomac River located in south central Pennsylvania and western Maryland in the United States, a region known as the Hagerstown Valley.

The course proceeds southward in a meandering pattern, and the creek empties into the Potomac south of Sharpsburg[2] about 50 miles (80 km) upstream of Washington.

Communities in the watershed include Waynesboro in Pennsylvania; and Boonsboro, Funkstown, Hagerstown, Mount Aetna, Sharpsburg and Smithsburg in Maryland.

Both sides lost about a fourth of their number[8] but, despite General McClellan's refusal to press on his attacks,[8] it served as a tactical Union victory, as Lee was forced to withdraw from Maryland.

The Maryland Department of the Environment (MDE) has identified farm runoff as the largest source of sediment (loose soil) in Antietam Creek and its tributaries.