Potassium nitrate is a chemical compound with the formula KNO3. It is an ionic salt of potassium ions K+ and nitrate ions NO3−.

It occurs as a mineral niter and is a natural solid source of nitrogen. Its common names include saltpetre (saltpeter in American English), from medieval Latin sal petræ: "stone salt" or possibly "Salt of Petra" and nitrate of potash. The mineral nitratite also named Chile saltpetre (American Chile saltpeter) refers not to potassium nitrate but to the natural mineral form of a similar chemical sodium nitrate.

Major uses of potassium nitrate are in fertilizers, food additive, rocket propellants and fireworks; it is one of the constituents of gunpowder