Hydroxide is a diatomic anion with chemical formula OH⁻. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a catalyst . The hydroxide ion form salts, which dissociate in aqueous solution, liberating solvated hydroxide ions. Sodium hydroxide is a multi-million-ton per annum commodity chemical. A hydroxide group attached to a strongly electropositive center may itself dissociate, liberating a hydrogen cation (H⁺), making the parent compound an acid.

In organic chemistry, the hydroxide ion can act as a catalyst or as a nucleophilic reagent. An OH group, known as an hydroxyl group, is present in alcohols, phenols, carboxylic acids and related functional groups.

The hydroxide ion is a natural constituent of water, because of the self-ionization reaction:

The equilibrium constant for this reaction, defined as

has a value close to 10⁻¹⁴ at 25 °C, so the concentration of hydroxide ions in pure water is close to 10⁻⁷ mol dm⁻³, in order to satisfy the equal charge constraint. The pH of a solution is equal to the decimal cologarithm of the hydrogen cation concentration; the pH of pure water is close to 7 at ambient temperatures. The concentration of hydroxide ions can be expressed in terms of pOH, which is close to 14 – pH, so pOH of pure water is also close to 7. Addition of a base to water will reduce the hydrogen cation concentration and therefore increase the hydroxide ion concentration (increase pH, decrease pOH) even if the base does not itself contain hydroxide. For example, ammonia solutions have a pH greater than 7 due to the reaction NH₃ + H⁺ NH₄⁺, which results in a decrease in hydrogen cation concentration and an increase in hydroxide ion concentration. pOH can be kept at a nearly constant value with various buffer solutions.