Hypophosphorous acid is a phosphorus oxoacid and a powerful reducing agent with molecular formula H₃PO₂. Inorganic chemists refer to the free acid by this name (also as "HPA"), or the acceptable name of phosphinic acid. It is a colorless low-melting compound, which is soluble in water, dioxane, and alcohols. The formula for hypophosphorous acid is generally written H₃PO₂, but a more descriptive presentation is HOP(O)H₂ which highlights its monoprotic character. Salts derived from this acid are called phosphinates (hypophosphites).

HOP(O)H₂ exists in equilibrium with the minor tautomer HP(OH)₂. Sometimes the minor tautomer is called hypophosphorous acid and the major tautomer is called phosphinic acid.

The acid is prepared industrially via a two step process. Hypophosphite salts of the alkali and alkaline earth metals result from treatment of white phosphorus with hot aqueous solution of the appropriate hydroxide, e.g. Ca(OH)₂.

The free acid may be prepared by the action of a strong acid on these hypophosphite salts.

Alternatively, H₃PO₂ arises by the oxidation of phosphine with iodine in water.

HPA is usually supplied as a 50% aqueous solution.

Its main industrial use is for electroless nickel plating (Ni–P), although it is primarily used as a salt (sodium hypophosphite). In organic chemistry, H₃PO₂ can be used for the reduction of arenediazonium salts, converting ArN+
2 to Ar–H. When diazotized in a concentrated solution of hypophosphorous acid, an amine substituent can be removed from arenes, selectively over alkyl amines. Owing to its ability to act as a mild reducing agent and oxygen scavenger it is sometimes used as an additive in Fischer esterification reactions, where it prevents the formation of colored impurities. Hypophosphorous acid is also used in the formulation of pharmaceuticals, discoloration of polymers, water treatment, retrieval of precious or non-ferrous metals.

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