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
See also 401 (number) 401 (disambiguation) 401, the year 401 (CDI) of the Julian calendar 401 BC