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. Potassium nitrate is one of several nitrogen-containing compounds collectively referred to as saltpeter.

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

The earliest known complete purification process for potassium nitrate was outlined in 1270 by the chemist and engineer Hasan al-Rammah of Syria in his book al-Furusiyya wa al-Manasib al-Harbiyya ('The Book of Military Horsemanship and Ingenious War Devices'). In this book, al-Rammah describes first the purification of barud (crude saltpetre mineral) by boiling it with minimal water and using only the hot solution, then the use of potassium carbonate (in the form of wood ashes) to remove calcium and magnesium by precipation of their carbonates from this solution, leaving a solution of purified potassium nitrate, which could then be dried. This was used for the manufacture of gunpowder and explosive devices. The terminology used by al-Rammah indicated a Chinese origin for the gunpowder weapons about which he wrote. While potassium nitrate was called "Chinese snow" by Arabs, it was called "Chinese salt" by the Iranians/Persians.