503rd or 503d may refer to:
503d Air Defense Group, inactive United States Air Force organization 503d Aircraft Control and Warning Group (AC&WG), inactive United States Air Force unit 503d Aircraft Control Group, inactive United States Air Force unit 503d Fighter Squadron, unit of the New York Air National Guard that flies the C-130 Hercules 503rd Heavy Panzer Battalion, German heavy Panzer Abteilung equipped with Tiger I tanks and Panzer IIIs during World War II 503rd Infantry Regiment (United States), an airborne forces regiment of the US Army 503rd SS Heavy Panzer Battalion or 103rd SS Heavy Panzer Battalion, originally formed as the II.Battalion, 11 SS Panzer Regiment
See also
No. 503 Squadron RAF, an Interwar British Royal Air Force bomber unit 503 Field Battery, Royal Artillery, British Army
503 (disambiguation), including ships with pennant no.503