S81 may refer to:
Aviation Blériot-SPAD S.81, a French fighter aircraft Indian Creek USFS Airport, in Valley County, Idaho, United States Short S.81, an experimental British biplane seaplane
Other uses S81 (New York City bus) serving Staten Island County Route S81 (Bergen County, New Jersey) Daihatsu Hijet (S81), a kei truck and microvan , a S-80 Plus class submarine Stola S81 Stratos, a concept car S81, a line of the St. Gallen S-Bahn S81, a postcode district for Worksop, England BenQ-Siemens S81, a BenQ mobile phone