Sulfur dioxide (also sulphur dioxide) is the chemical compound with the formula SO₂. It is a poisonous gas with a pungent, irritating smell, that is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel. Further oxidation of SO₂, usually in the presence of a catalyst such as NO₂, forms H₂SO₄, and thus acid rain. Sulfur dioxide emissions are also a precursor to particulates in the atmosphere. Both of these impacts are cause for concern over the environmental impact of these fuels.

SO₂ is a bent molecule with C_2vsymmetry point group. In terms of electron-counting formalism, the sulfur atom has an oxidation state of +4 and a formal charge of 0. It is surrounded by 5 electron pairs and can be described as a hypervalent molecule. From the perspective of molecular orbital theory, most of these valence electrons are engaged in S–O bonding.