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

* , 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

Roads

Mehran Highway Mehran Highway ( sd, مِهِراڻ ُ هاءِوي, ur, , also known locally as Nawabshah–Kot Lalu–Kumb Road and officially as Highway 31) is a two-lane-wide, bidirectional, single carriageway, provincially maintained highway in Sindh t ...

, 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 {{Letter-NumberCombDisambig