Saltpeter, the common name for potassium nitrate, a white, crystalline solid composed of potassium, nitrogen, and oxygen. It is also called niter. Saltpeter commonly occurs as a crust on the soil and on the surface of rocks in dry climates and in the soil of limestone caves. Its name is derived from the Latin sal petrae, salt of the rock. Naturally occurring saltpeter is found in very limited quantities. Most saltpeter is produced commercially by the reaction of potassium chloride with sodium nitrate.

Saltpeter is used as an oxidizer in explosives, matches, fireworks, and rocket propellants. It is also used as a source of potassium and nitrogen in fertilizers and as a meat preservative.

Chile saltpeter is a form of sodium nitrate. It resembles saltpeter in appearance, and sometimes occurs in the same deposits as saltpeter. The largest deposits of Chile saltpeter are found in the desert regions of northern Chile and Bolivia. Chile saltpeter is used in fertilizers, in explosives, and in the manufacture of nitric acid. After World War I synthetic processes were developed for producing large quantities of sodium nitrate, so the demand for the natural form has declined.

The formula for saltpeter is KNO3; for Chile saltpeter, NaNO3.