51st is the ordinal form of the number 51. 51st or Fifty-first may also refer to:
A fraction, , equal to one of 51 equal parts The 51st, nonprofit local news website registered in the District of Columbia
Geography 51st meridian east, a line of longitude 51st meridian west, a line of longitude 51st parallel north, a circle of latitude 51st parallel south, a circle of latitude 51st station, on the Chicago Transit Authority's "L" system 51st state
Military 51st Army (disambiguation) 51st Brigade (disambiguation) 51st Division (disambiguation) 51st Regiment (disambiguation) 51st Squadron (disambiguation)
Other 51st century 51st century BC
See also 51 (disambiguation)