81st is the ordinal form of the number 81. 81st or Eighty-first may also refer to:
A fraction, , equal to one of 81 equal parts
Geography 81st meridian east, a line of longitude 81st meridian west, a line of longitude 81st parallel north, a circle of latitude 81st parallel south, a circle of latitude 81st Street (disambiguation)
Military 81st Group Army, People's Republic of China 81st Brigade (disambiguation) 81st Division (disambiguation) 81st Regiment (disambiguation) 81st Squadron (disambiguation)
Other 81st century 81st century BC
See also 81 (disambiguation) The 81st Blow , 1974 Israeli documentary film