S21

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

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

 * ARA Santa Fe, of the Argentine Navy
 * BRAZILIAN SUBMARINE Tonelero
 * HMS Onyx (S21), of the Royal Navy
 * INS Kalvari (S21), of the Indian Navy
 * INS Karanj (S21), of the Indian Navy
 * USS S-21 (SS-126), 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
 * Forward voltage gain or transmission coefficient $$S_{21}$$ of a two-port network
 * 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
 * s21 notice, in England & Wales, from a landlord instructing tenants to leave