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 Bombardment Squadron, inactive United States Air Force unit
 * 401st Brigade (IDF), an Armored Brigade in the 162nd Division of the Israeli Defence Forces (IDF)
 * 401st Fighter Squadron, inactive United States Air Force unit