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
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