S51 may refer to:
Aircraft Blériot-SPAD S.51, a French biplane fighter SIAI S.51, an Italian racing flying boat Sikorsky S-51, an American helicopter
Other uses S51 (New York City bus) serving Staten Island S51 (RER Fribourg), a rail line in Switzerland County Route S51 (Bergen County, New Jersey) Nikon Coolpix S51, a digital camera S51: Use only in well-ventilated areas, a safety phrase S-51 Self-Propelled Gun, a Soviet experimental howitzer Shorland S51, a British armoured car Tswa language , a submarine of the United States Navy Toyota S51, a Toyota S transmission