91st is the ordinal form of the number 91. 91st or Ninety-first may also refer to:
A fraction, , equal to one of 91 equal parts
Geography 91st meridian east, a line of longitude 91st meridian west, a line of longitude 91st Street (disambiguation)
Military 91st Brigade (disambiguation) 91st Division (disambiguation) 91st Regiment (disambiguation)
Other 91st century 91st century BC
See also 91 (disambiguation) April 1, 91st day of the year in a standard year