Octasulfur is an inorganic substance with the chemical formula S₈. It is an odourless and tasteless yellow solid, and is a major industrial chemical. It is the most common allotrope of sulfur and occurs widely in nature.^[4]

The name octasulfur is the most commonly used for this chemical. It is systematically named cyclo-octasulfur (which is the preferred IUPAC name) and cyclooctasulfane. It is also the final member of the thiocane heterocylic series, where every carbon atom is substituted with a sulfur atom, thus this sulfur allotrope is systematically named octathiocane as well.

The chemical consists of rings of 8 sulfur atoms. It adopts a crown conformation with D_4d point group symmetry. The S–S bond lengths are equal, at about 2.05 Å. Octasulfur crystallizes in three distinct polymorphs: rhombohedral, and two monoclinic forms, of which only two are stable at standard conditions. The rhombohedral crystal form is the accepted standard state. The remaining polymorph is only stable between 96 and 115 °C at 100 kPa. Octasulfur forms several allotropes: α-sulfur, β-sulfur, γ-sulfur, and λ-sulfur.

λ-Sulfur is the liquid form of octasulfur, from which γ-sulfur can be crystallised by quenching. If λ-sulfur is crystallised slowly, it will revert to β-sulfur. Since it must have been heated over 115 °C, neither crystallised β-sulfur or γ-sulfur will be pure. The only known method of obtaining pure γ-sulfur is by crystallising from solution.

Octasulfur easily forms large crystals, which are typically yellow and are somewhat translucent.

Octasulfur is not typically produced as S₈ per se. It is the main (99%) component of elemental sulfur, which is recovered from volcanic sources and is a major product of the Claus process, associated with petroleum refineries.

• Sulfur (pharmacy)