S31

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, a submarine of the Argentine Navy
 * BRAZILIAN SUBMARINE Tamoio
 * 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
 * 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