S21

S21 or S-21 may be:

• S-21: The Khmer Rouge Killing Machine, a 2003 documentary film by Rithy Panh
• Tuol Sleng Genocide Museum, the former Security Prison 21, or S-21, under the Khmer Rouge in Cambodia.
• S21 postcode, covering areas of North East Derbyshire
• S21: When using do not smoke, a safety phrase in chemistry
• British NVC community S21, a swamps and tall-herb fens community in the British National Vegetation Classification system
• Spalinger S-21, a Swiss glider designed by Jakob spalinger

S₂₁ may refer to :

• S-parameters S₂₁, forward transmission coefficient (gain)

Transportation

• Hamburg S-Bahn S21, from Elbgaustraße to Aumühle
• County Route S21 (California)
• San Diego County Route S21, the historic coastal highway
• Shin-Fukae Station, S21 on the Osaka Municipal Subway Sennichimae Line
• S21 (ZVV), a S-Bahn line in the cantons of Zürich and Zug in Switzerland
• The FAA identifier for Sunriver Airport in Sunriver, Oregon
• Stuttgart 21, a transportation and urban project in Stuttgart, Germany

Vehicles and Vessels

• Brazilian submarine Tonelero (S21)
• HMS Onyx (S21)
• USS S-21 (SS-126), a 1920 S-class submarine of the United States Navy
• Sukhoi-Gulfstream S-21, a proposed supersonic business jet