121 is the natural number following 120 and preceding 122.

One hundred twenty-one is a square and is the sum of three consecutive primes (37 + 41 + 43). It is also a star number and a centered octagonal number. In base 10, it is a Smith number since its digits add up to the same value as its factorization (which uses the same digits) and as a consequence of that it is a Friedman number. But it can not be expressed as the sum of any other number plus that number's digits, making 121 a self number.

One hundred twenty-one is also:

• The number for voicemail for mobile phones on the Vodafone network
• The model number of the Lockheed EC-121, now on display at the Air Force Museum .
• The year AD 121 or 121 BC.