129 (one hundred [and] twenty-nine) is the natural number following 128 and preceding 130.

129 is the sum of the first ten prime numbers. It is the smallest number that can be expressed as a sum of three squares in four different ways: ${\displaystyle 11^{2}+2^{2}+2^{2}}$, ${\displaystyle 10^{2}+5^{2}+2^{2}}$, ${\displaystyle 8^{2}+8^{2}+1^{2}}$, and ${\displaystyle 8^{2}+7^{2}+4^{2}}$.

129 is the product of only two primes, 3 and 43, making 129 a semiprime. Since 3 and 43 are both Gaussian primes, this means that 129 is a Blum integer.

129 is a repdigit in base 6 (333).

129 is a happy number.

129 is a centered octahedral number.