A halonium ion in organic chemistry is any onium compound (ion) containing a bridged halogen atom carrying a positive charge. This cation has the general structure R-X⁺-R where X is any halogen and R any organic residue and this structure can be cyclic or an open chain molecular structure.

Halonium ions were first postulated in 1937 by Roberts and Kimball ^[1] to account for observed diastereoselectivity in halogen addition reactions to alkenes. They correctly argued that if the initial reaction intermediate in bromination is the open-chain X^–-C-C⁺, rotation around the C-C single bond would be possible leading to a mixture of equal amounts of dihalogen cis isomer and trans isomer which is not the case. They also asserted that a positively charged halogen atom is isoelectronic with oxygen and that carbon and bromine have comparable ionization potentials.

In 1970 George A. Olah succeeded in preparing and isolating halonium salts ^[2] by adding a methyl halide such as methyl bromide or methyl chloride in sulfur dioxide at -78°C to a complex of antimony pentafluoride and tetrafluoromethane in sulfur dioxide. After evaporation of sulfur dioxide this procedure left crystals of CH₃-X⁺-CH₃SbF₆^–, stable at room temperature but not to moisture.

A halonium ion also seems to refer generically to the simpler onium compounds (ions) based on halogens: fluoronium, H₂F⁺; chloronium, H₂Cl⁺; bromonium, H₂Br⁺; iodonium, H₂I⁺.

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