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

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

• ARA Santa Fe (S-21), of the Argentine Navy
• Brazilian submarine Tonelero (S21)
• 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