R21

R21, R-21 or R.21 may refer to:

In military 

• HMAS Melbourne (R21), a 1945 Australian Royal Navy ship of the Majestic class of light aircraft carrier
• R-21 (missile), a submarine-launched ballistic missile developed and deployed by the Soviet Union
• USS R-21 (SS-98), a 1918 R-class coastal and harbor defense submarine of the United States Navy

In transportation 

• R21 (New York City Subway car)
• R21 road (Senegal)
• R21 road (South Africa)
• Renault 21, a large family car produced between 1986 and 1994

In science and academia 

• R21: Harmful in contact with skin, a risk phrase in chemistry
• Dichlorofluoromethane, a hydrochlorofluorocarbon used as a propellant and refrigerant
• NIH Exploratory/Developmental Research Grant Award, a type of United States National Institute of Health grant