William Quantrill, American captain (d. 1865)
William Clarke Quantrill (July 31, 1837 – June 6, 1865) was a Confederate guerrilla leader during the American Civil War.

Having endured a tempestuous childhood before later becoming a schoolteacher, Quantrill joined a group of bandits who roamed the Missouri and Kansas countryside to apprehend escaped slaves. Later, the group became Confederate soldiers, who were referred to as "Quantrill's Raiders". It was a pro-Confederate partisan ranger outfit that was best known for its often brutal guerrilla tactics. Also notable is that the group included the young Jesse James and his older brother Frank James.

Quantrill is often noted as influential in the minds of many bandits, outlaws and hired guns of the Old West as it was being settled. In May 1865, Quantrill was mortally wounded in combat by Union troops in Central Kentucky in one of the last engagements of the Civil War. He died of wounds in June.