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), formerly the 503rd Parachute Infantry Regiment (PIR), an airborne unit in the United States military
• 503rd SS Heavy Panzer Battalion or 103rd SS Heavy Panzer Battalion, originally formed as the II.Battalion, 11 SS Panzer Regiment

• 503 (number)
• 503, the year 503 (DIII) of the Julian calendar
• 503 BC