M103 or M-103 may refer to: M103 heavy tank, an American heavy tank M103 (New York City bus), a bus route in Manhattan M-103 (Michigan highway), a state highway in Michigan Mercedes-Benz M103 engine Messier 103, an open star cluster in the constellation Cassiopeia The ADC ID of Lyndonville Air Force Station Motion 103, also known as M-103; a motion in the House of Commons of Canada. M103 railway (Croatia), railway line in Croatia