451st

451st may refer to:


 * 451st Air Expeditionary Wing, a provisional United States Air Force Air Combat Command unit, currently in Afghanistan
 * 451st Expeditionary Sustainment Command (ESC) is a subordinate command of 79th Sustainment Support Command
 * 451st Flying Training Squadron, active United States Air Force unit
 * 451st Intelligence Squadron, intelligence unit located at RAF Menwith Hill, United Kingdom