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The nitrate ion is a polyatomic ion with the molecular formula NO− 3 and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a formal charge of -1. This results from a combination formal charge in which each of the three oxygens carries a −2⁄3 charge, whereas the nitrogen carries a +1 charge, all these adding up to formal charge of the polyatomic nitrate ion.

This arrangement is commonly used as an example of resonance. Like the isoelectronic carbonate ion, the nitrate ion can be represented by resonance structures:

Almost all inorganic nitrate salts are soluble in water at standard temperature and pressure. A common example of an inorganic nitrate salt is potassium nitrate (saltpetre).

In organic chemistry a nitrate (not to be confused with nitro) is a functional group with general chemical formula RONO2 where R stands for any organic residue. They are the esters of nitric acid and alcohols formed by nitroxylation. Examples are methyl nitrate formed by reaction of methanol and nitric acid, the nitrate of tartaric acid, and the inaccurately-named nitroglycerin (which is actually an organic nitrate compound, not a nitro compound).

Like organic nitro compounds (see below) both organic and inorganic nitrates can be used as propellants and explosives. An example of the use of inorganic nitrate was classical gunpowder. In all these uses the thermal decomposition of the nitrate yields molecular nitrogen N2 gas plus considerable chemical energy, due to the high strength of the bond in molecular nitrogen. Especially in inorganic nitrate reactions, oxidation from the nitrate oxygens is also an important energy-releasing process.

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