The 21st Expeditionary Reconnaissance Squadron is a provisional United States Air Force unit, assigned to United States Air Forces Europe to activate or inactivate as needed. Its last known location was at Souda Bay, Greece. The squadron was active as the 21st Tactical Reconnaissance Squadron, and served as a reconnaissance Replacement Training Unit during World War II until being disbanded in a reorganization of the United States Army Air Forces designed to make the most efficient use of resources to free manpower for overseas assignment.