Niter (AmE) or nitre (BrE) is the mineral form of potassium nitrate, KNO3, also known as saltpeter (AmE) or saltpetre (BrE). Historically, the term "niter" – cognate with "natrium", a Latin word for sodium – has been very vaguely defined, and it has been applied to a variety of other minerals and chemical compounds, including sodium nitrate (also "soda niter", "cubic niter", or nitratite), sodium carbonate and potassium carbonate.

Because of its ready solubility in water, niter is most often found in arid environments. A major source of sodium (not potassium) nitrate mineral is the Atacama desert in Chile. Potassium and other nitrates are of great importance for use in fertilizers, and, historically, gunpowder. Much of the world's demand is now met by synthetically produced nitrates, though the natural mineral is still mined and is still of significant commercial value.