Matthias W. Day (August 8, 1853 – September 12, 1927) was a career American army officer who received the Medal of Honor, the United States' highest military decoration, for his actions during the American Indian Wars in the latter half of the 19th century. Day was a longtime officer with the African-American 9th Cavalry Regiment, seeing action during the Apache Wars against the Apache leaders Victorio and Geronimo.