S31 may refer to:
Aviation Blériot-SPAD S.31, a French racing aircraft Letov Š-31, a Czechoslovak fighter aircraft Lopez Island Airport, in Washington, United States Short S.31, a British bomber prototype Sikorsky S-31, an American sesquiplane
Naval vessels , a submarine of the Argentine Navy
, a submarine of the Royal Navy , a torpedo boat of the Imperial German Navy
, 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 S31, a line of the Lucerne S-Bahn, in Switzerland S31, a former line of the Zurich S-Bahn, in Switzerland
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