S51

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 Fribourg, 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
 * USS S-51 (SS-162), a submarine of the United States Navy
 * Toyota S51, a Toyota S transmission