The 503rd Infantry Regiment, formerly the 503rd Parachute Infantry Regiment (503rd PIR) and the 503rd Airborne Infantry Regiment (503rd AIR), is an airborne infantry regiment of the United States Army.
Resimen Infanteri ke-503 (503rd Infantry Regiment), dulunya Resimen Infanteri Parasut ke-503 (503rd Parachute Infantry Regiment, PIR) dan Resimen Infanteri Udara ke-503 (503rd Airborne Infantry Regiment, AIR), adalah sebuah resimen infanteri angkatan udara dari Angkatan Darat Amerika Serikat.