In chemistry, a hydronium ion is the cation H₃O⁺, a type of oxonium ion produced by protonation of water and isoelectronic with ammonia. This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated (bound to a solvent). The reality is far more complicated, and a proton is bound to several molecules of water, such that other descriptions such as H₅O₂⁺, H₇O₃⁺ and H₉O₄⁺ are increasingly accurate descriptions of the environment of a proton in water. The ion H₃O⁺ has been detected in the gas phase.

It is the presence of hydronium ion relative to hydroxide that determines a solution's pH. Water molecules auto-dissociate into hydronium and hydroxide ions in the following equilibrium:

In pure water, there is an equal number of hydroxide and hydronium ions. At 25 °C and atmospheric pressure their concentrations are approximately equal to 1.0 × 10⁻⁷ mol∙dm⁻³. For these conditions, [H₃O⁺] = 10^−pH so pH = 7 is defined as neutral. A pH value less than 7 indicates an acidic solution, and a pH value more than 7 indicates a basic solution. Note that [H₃O⁺]×[OH⁻], the ionic product of water, strongly increases with temperature so [H₃O⁺] is not equal to 10^−pH for temperatures other than 25 °C.