401st

401st may refer to:


 * 401st Air Expeditionary Group, provisional United States Air Force unit assigned to the United States Air Forces in Europe
 * 401st Air Expeditionary Wing, inactive unit of the United States Air Force
 * 401st Army Field Support Brigade, forward support brigade of the United States Army
 * 401st Brigade, an armored unit of the Israeli Defence Forces
 * 401st Squadron (disambiguation), several military units