S31
S31 may refer to :
- S31 (Long Island bus)
- County Route S31 (California)
- County Route S31 (New Jersey), a county route in Bergen County
- HMS Vengeance (S31), a 1998 Vanguard class submarine of the British Royal Navy
- Letov Š-31, a Czekoslovakian Letov aircraft
- USS S-31 (SS-136), a 1918 S-class submarine of the United States Navy
- S-31, a WW2 German Schnellboot
- a Hamburg S-Bahn line
- a Stadtbahn Karlsruhe line