Special Order 191, also known as the "Lost Dispatch" and the "Lost Order", was a general movement order issued by Confederate Army General Robert E. Lee on about September 9, 1862, during the Maryland Campaign of the American Civil War. A lost copy of this order was recovered on September 13 by Union Army troops in Frederick County, Maryland, and the subsequent military intelligence gained by the Union played an important role in the Battle of South Mountain and Battle of Antietam.

The Maryland campaign occurred September 4–20, 1862, during the American Civil War. The campaign was Confederate General Robert E. Lee's first invasion of the North. It was repulsed by the Army of the Potomac under Maj. Gen. George B. McClellan, who moved to intercept Lee and his Army of Northern Virginia and eventually attacked it near Sharpsburg, Maryland. The resulting Battle of Antietam was the bloodiest day of battle in American history.