Hannibal Day (February 15, 1804 – March 26, 1891) served as an officer in the United States Army. His career spanned 46 years, and included the Black Hawk War, Seminole War, Mexican–American War, and American Civil War. One of the oldest officers to serve in the Union Army, Day commanded a brigade in the Army of the Potomac and took part in combat at the Battle of Gettysburg.