S31 may refer to:
Aviation
- Blériot-SPAD S.31, a French racing aircraft
- Letov Š-31, a Czechoslovakian fighter aircraft
- Lopez Island Airport, in San Juan County, Washington, United States
- Short S.31, a British bomber prototype
- Sikorsky S-31, an American sesquiplane
Naval vessels
- ARA Salta (S-31), a submarine of the Argentine Navy
- Brazilian submarine Tamoio (S-31)
- HMS Vengeance (S31), a submarine of the Royal Navy
- SMS S31 (1913), a torpedo boat of the Imperial German Navy
- Spanish submarine Almirante García de los Reyes (S31)
- USS S-31 (SS-136), a submarine of the United States Navy
Rail and transit
- S31 (Long Island bus)
- Warabitai Station, a closed station in Oshamambe, Hokkaido, Japan
- S31, a line of the Bern S-Bahn, in Switzerland
- S31, a line of the Hamburg S-Bahn, in Germany
- S31, a line of the Karlsruhe Stadtbahn, in Germany
Roads
- Mehran Highway, in Sindh, Pakistan
- County Route S31 (California), United States
- County Route S31 (Bergen County, New Jersey), United States
- U.S. Route 206, partially numbered New Jersey Route S31 until 1953
Other uses
- Sulfur-31, an isotope of sulfur
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