John Gross Barnard (May 19, 1815 – May 14, 1882) was a career engineer officer in the U.S. Army, serving in the Mexican–American War, as the superintendent of the United States Military Academy and as a general in the Union Army during the American Civil War. He served as Chief Engineer of the Army of the Potomac, 1861 to 1862, Chief Engineer of the Department of Washington from 1861 to 1864, and as Chief Engineer of the armies in the field from 1864 to 1865. He also was a distinguished scientist, engineer, mathematician, historian and author.