Sulfur dioxide (SO₂) is an inorganic triatomic species that has been identified in the interstellar medium, at least one comet, and in several planetary and satellite atmospheres in our solar system, most notably Venus and Io. Given that Io is covered with sulfur "frost" from its unique volcanos, it is not surprising that SO₂ dominates its rarified atmosphere. Sulfur dioxide has also been observed on Io frozen in ice.

SO₂ is observed via both radio and infrared astronomy. With the latter, it has been detected in both the gas and condensed phases.

Sulfur is one of the biogenic elements, along with carbon, hydrogen, nitrogen, oxygen, and phosphorous. It is a component of amino acids such as cysteine and methionine.