121 may refer to:
121 (number), a natural number
AD 121, a year in the 2nd century AD
121 BC, a year in the 2nd century BC
121 (Eagle) Sqn, a Royal Air Force aircraft squadron that during the Second World War was one of the three Eagle Squadrons manned by American volunteers
121 (MBTA bus), a Massachusetts Bay Transport Authority bus route
121 (New Jersey bus), a New Jersey Transit bus route
Road 121, see list of highways numbered 121
Russian cruiser Moskva (pennant number: 121)
"121", a song by Robert Forster from the album Calling from a Country Phone
121 Hermione, a main-belt asteroid
Mazda 121, various models of automobiles produced by Mazda