S21 or S-21 may refer to: Automobiles Chery S21, a city car Sisu S-21, a Finnish lorry Aviation De Schelde S.21, a proposed Dutch fighter Rans S-21 Outbound, an American homebuilt aircraft SIAI S.21, an Italian racing flying boat Sikorsky S-21, the first four-engine aircraft in the world Short S.21 Maia, a British flying boat mother ship Spalinger S.21, a Swiss training glider Sukhoi-Gulfstream S-21, a proposed American business jet Sunriver Airport in Sunriver, Oregon Rail and transit Lines S21 (Berlin), a planned S-Bahn line in Berlin, Germany S21 (RER Fribourg), an S-Bahn line in the canton of Freibourg, Switzerland S21 (ZVV), an S-Bahn line in the cantons of Zürich and Zug in Switzerland Stuttgart 21, a transportation and urban project in Stuttgart, Germany S21, a line of the Hamburg S-Bahn Stations Ginzan Station, in Niki, Hokkaidō, Hokkaidō, Japan Moto-Yawata Station, in Ichikawa, Chiba, Japan Shin-Fukae Station, in Osaka, Japan Tokushige Station, in Midori-ku, Nagoya, Aichi, Japan Roads S21 Guangzhou–Huidong Expressway, China County Route S21 (California), United States Submarines , of the Argentine Navy , of the Royal Navy , of the Indian Navy , of the Indian Navy , of the United States Navy Other uses 40S ribosomal protein S21 British NVC community S21, a swamps and tall-herb fens community in the British National Vegetation Classification system S-21: The Khmer Rouge Killing Machine, a 2003 documentary film S21: When using do not smoke, a safety phrase Samsung Galaxy S21, a series of smartphones Security Prison 21, now the Tuol Sleng Genocide Museum in Cambodia Serbia 21, a political organization in Serbia