Marcus Albert Reno (November 15, 1834 – March 30, 1889) was a career military officer in the American Civil War and in the Great Sioux War against the Lakota (Sioux) and Northern Cheyenne, where he served under George Armstrong Custer. Reno is most noted for his prominent role in the Battle of the Little Big Horn, which created over a century of controversy regarding his command decisions in the course of one of the most infamous defeats in the history of the United States military.