Thirty-one (31, XXXI) is the natural number following thirty and preceeding thirty-two. It is a prime number, the third Mersenne prime ( 2⁵ - 1 ) as well as a primorial prime, and together with twenty-nine, another primorial prime, it comprises a twin prime. As a Mersenne prime, 31 is related to the perfect number 496, since 496 = 2^{5 - 1} ( 2⁵ - 1).

Thirty-one is also:

• The atomic number of gallium.
• The number of days in the months January, March, May, July, August, October and December.
• The code for international direct dial phone calls to the Netherlands.
• A type of card game. See Thirty-one (game).
• The number of flavors of Baskin Robbins.
• In Star Trek, the name of Section 31, a secret police shadow organization of the Federation.