Thirty-nine (39, XXXIX) is the natural number following thirty-eight and preceeding forty. Its divisors are 1, 3, 13, and 39; its factorisation is

This number is the sum of five consecutive primes (3 + 5 + 7 + 11 + 13).

• The atomic number of yttrium.
• In the title of the John Buchan, novel The Thirty-Nine Steps.
• The code for international direct-dialed phone calls to Italy.
• The eternal age of comedian Jack Benny, and many other people as they grow older.