400th may refer to:
- 400th anniversary of the Authorized King James Version, set of events and exhibitions in 2011
- 400th Bombardment Group, inactive United States Air Force unit
- 400th Missile Squadron, inactive United States Air Force unit
- America's 400th Anniversary or Jamestown 2007
See also
- 400 (number)
- 400, the year 400 (CD) of the Julian calendar
- 400 BC
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