399th may refer to:
399th Bombardment Group, inactive United States Air Force unit 399th Bombardment Squadron or 99th Air Refueling Squadron, part of the 6th Air Mobility Wing at Birmingham Air National Guard Base, Alabama 399th Fighter Squadron or 57th Weather Reconnaissance Squadron, inactive United States Air Force Reserve squadron
See also 399 (number) 399, the year 399 (CCCXCIX) of the Julian calendar 399 BC