R21, R-21 or R.21 may refer to:
Military R-21 (missile), a Soviet submarine-launched ballistic missile , an aircraft carrier of the Royal Australian Navy , a destroyer of the Royal Navy , a submarine of the United States Navy
Roads R-21 regional road (Montenegro) R21 (South Africa) R21 highway (Russia)
Science and medicine Dichlorofluoromethane, a refrigerant R21: Harmful in contact with skin, a risk phrase R21/Matrix-M a Malaria vaccine Small nucleolar RNA R21
Other uses R21 (New York City Subway car) Kwanyama dialect Renault 21, a French family car Tumansky R-21, a Soviet turbojet engine