硝石，也称消石、火硝、牙硝（古书上又称茫消或北帝玄珠，焰硝等），是一种天然矿物，主要成分为硝酸钾。

其他来源的硝酸钾，以及其他的硝酸盐矿物如智利硝石（硝酸钠）、挪威硝石（硝酸钙）有的时候也被称作硝石。但注意芒硝不是硝酸盐，而是含结晶水的硫酸钠。

硝石为透明或白色晶体，属斜方晶系。其摩氏硬度与石膏接近，比重2.1, 折射率 nα=1.332, nβ=1.504, nγ=1.504。硝石易溶于水。天然硝石主要见于碱土地区的干燥土壤中，矿泉、和洞穴壁上，由富含硝酸钾的水常年浸润生成。

Niter, or nitre ("chiefly British variant"), is the mineral form of potassium nitrate, KNO₃, also known as saltpeter or saltpetre. Historically, the term "niter" was not well differentiated from natron, both of which have been very vaguely defined but generally refer to compounds of sodium or potassium joined with carbonate or nitrate ions. Three related minerals are soda niter (sodium nitrate), ammonia niter (ammonium nitrate), and strontium nitrate. Niter was used to refer specifically to nitrated salts known as various types of saltpeter (only nitrated salts were good for making gunpowder) by the time niter and its derivative nitric acid were first used to name the element nitrogen, in 1790.