Ozone (/ˈoʊzoʊn/ ^ⓘ), also called trioxygen, is an inorganic molecule with the chemical formula O
₃. It is a pale-blue gas with a distinctively pungent odor. It is an allotrope of oxygen that is much less stable than the diatomic allotrope O
₂, breaking down in the lower atmosphere to O
₂ (dioxygen). Ozone is formed from dioxygen by the action of ultraviolet (UV) light and electrical discharges within the Earth's atmosphere. It is present in very low concentrations throughout the atmosphere, with its highest concentration high in the ozone layer of the stratosphere, which absorbs most of the Sun's ultraviolet (UV) radiation.

Ozone's odor is reminiscent of chlorine, and detectable by many people at concentrations of as little as 0.1 ppm in air. Ozone's O₃ structure was determined in 1865. The molecule was later proven to have a bent structure and to be weakly diamagnetic. At standard temperature and pressure, ozone is a pale blue gas that condenses at cryogenic temperatures to a dark blue liquid and finally a violet-black solid. Ozone's instability with regard to more common dioxygen is such that both concentrated gas and liquid ozone may decompose explosively at elevated temperatures, physical shock, or fast warming to the boiling point. It is therefore used commercially only in low concentrations.

Ozone is a powerful oxidizing agent (far more so than dioxygen) and has many industrial and consumer applications related to oxidation. This same high oxidizing potential, however, causes ozone to damage mucous and respiratory tissues in animals, and also tissues in plants, above concentrations of about 0.1 ppm. While this makes ozone a potent respiratory hazard and pollutant near ground level, a higher concentration in the ozone layer (from two to eight ppm) is beneficial, preventing damaging UV light from reaching the Earth's surface.

The trivial name ozone is the most commonly used and preferred IUPAC name. The systematic names 2λ⁴-trioxidiene^{[dubious – discuss]} and catena-trioxygen, valid IUPAC names, are constructed according to the substitutive and additive nomenclatures, respectively. The name ozone derives from ozon (ὄζον), the Greek neuter present participle of ozein (ὄζειν) "to smell", referring to ozone's distinctive smell.

In appropriate contexts, ozone can be viewed as trioxidane with two hydrogen atoms removed, and as such, trioxidanylidene may be used as a systematic name, according to substitutive nomenclature. By default, these names pay no regard to the radicality of the ozone molecule. In an even more specific context, this can also name the non-radical singlet ground state, whereas the diradical state is named trioxidanediyl.

Trioxidanediyl (or ozonide) is used, non-systematically, to refer to the substituent group (-OOO-). Care should be taken to avoid confusing the name of the group for the context-specific name for the ozone given above.

In 1785, Dutch chemist Martinus van Marum was conducting experiments involving electrical sparking above water when he noticed an unusual smell, which he attributed to the electrical reactions, failing to realize that he had in fact produced ozone.

A half century later, Christian Friedrich Schönbein noticed the same pungent odour and recognized it as the smell often following a bolt of lightning. In 1839, he succeeded in isolating the gaseous chemical and named it "ozone", from the Greek word ozein (ὄζειν) meaning "to smell". For this reason, Schönbein is generally credited with the discovery of ozone. He also noted the similarity of ozone smell to the smell of phosphorus, and in 1844 proved that the product of reaction of white phosphorus with air is identical. A subsequent effort to call ozone "electrified oxygen" he ridiculed by proposing to call the ozone from white phosphorus "phosphorized oxygen". The chemical formula for ozone, O₃, was not determined until 1865 by Jacques-Louis Soret and confirmed by Schönbein in 1867.