William Selby Harney (22 August 1800 - 9 May 1889) was a cavalry officer in the U.S. Army during the Mexican-American War and the Indian Wars.

Harney started his military career in 1818 as a second lieutenant in the 1st U.S. Infantry and served with distinction during the Seminole Wars, the Blackhawk War, the Mexican-American War, and the Indian Wars. At the opening of the Civil War he was one of four general officers in the regular army.

In Saint Louis, Missouri, where he commanded the Army's Department of the West, he warned that state against secession, but raised the suspicions of political enemies when he negotiated with Confederate General Sterling Price in order to keep the state from being torn in two by the war. Called to Washington, he was captured by Confederates enroute and was offered a command by Confederate General Robert E. Lee. He refused and was released to continue on to Washington, where he was shocked to learn that he was being relieved of his command. He retired in 1863 and was breveted to Major General in 1865 in recognition of his long and distinguished career. President Lincoln said that the removal of General Harney was one of the greatest mistakes of his administration.

Though a then-well-known cavalry officer of the Indian Wars, William Harney worked for peace with the Indians by advocating a good neighbor policy and strove throughout his career to improve the nation's treatment of the native population, vainly seeking to have them treated fairly. The Crows gave him the name "Man-who-runs-like-the-deer" after he challenged them to foot races outside the walls of the fort. After the Civil War he was a key figure in the Indian Peace Commission that negotiated treaties with all the Plains Indians in 1867-68, and urged Congress to honor past treaties. After his death in Orlando, Florida, the Sioux changed his name to "Man-who-always-kept-his-word".

Harney was commander at the Grattan Massacre in 1855. For his merciless handling of the battle he was known among the Sioux as "the Butcher."

Harney's home in Sullivan, Mo. is privately owned by an organization that is seeking its restoration.