517th

517th may refer to:


 * 517th Air Defense Group, disbanded United States Air Force organization
 * 517th Airlift Squadron (517 AS), part of the 3d Wing at Elmendorf Air Force Base, Alaska
 * 517th Parachute Infantry Regiment (United States) (517th PIR), separate infantry regiment of the United States Army
 * 517th Parachute Regimental Combat Team (517th PRCT), one of the U.S. Army's first elite combat units
 * 517th Strategic Fighter Squadron, inactive United States Air Force unit