Thomas Horn Jr. (November 21, 1860 – November 20, 1903) was an American scout, cowboy, soldier, range detective, and Pinkerton agent in the 19th-century and early 20th-century American Old West. Believed to have committed 17 killings as a hired gunman throughout the West, Horn was convicted in 1902 of the murder of 14-year-old Willie Nickell near Iron Mountain, Wyoming. Willie was the son of sheep rancher Kels Nickell, who had been involved in a range feud with neighbor and cattle rancher Jim Miller. On the day before his 43rd birthday, Horn was executed by hanging in Cheyenne, Wyoming.
While in jail he wrote his autobiography, Life of Tom Horn: Government Scout and Interpreter, which was published posthumously in 1904. Numerous editions have been published in the late 20th century. Horn has since become a larger-than-life figure of western folklore, and debate continues as to whether he was actually guilty of Nickell's murder.