139 (one hundred [and] thirty-nine) is the natural number following 138 and preceding 140.
Because 141 is a semiprime, 139 is a Chen prime.
It is the smallest factor of 64079 which is the smallest Lucas number with prime index which is not prime.
It is also the smallest factor of the first nine terms of the Euclid–Mullin sequence, making it the tenth term.
139 is a happy number[2] and a strictly non-palindromic number.