S21
- 40S ribosomal protein S21
- Appenzell–St. Gallen–Trogen railway
- Chery S21
- County Route S21 (California)
- De Schelde S.21
- Ginzan Station
- Hamburg S-Bahn
- Moto-Yawata Station
- Rans S-21 Outbound
- S21 (Berlin)
- S21 (RER Fribourg)
- S21 (ZVV)
- S21 Guangzhou–Huidong Expressway
- SIAI S.21
- Samsung Galaxy S21
- Security Prison 21
- Serbia 21
- Shin-Fukae Station
- Short S.21 Maia
- Sikorsky S-21
- Sisu S-21
- Spalinger S.21
- Stuttgart 21
- Sukhoi-Gulfstream S-21
- Sunriver Airport
- Tokushige Station