Iron(III) sulfide, also known as ferric sulfide or sesquisulfide (Fe2S3), is one of the several binary iron sulfides.

It is a solid, black powder that degrades at ambient temperature.

[2] Fe2S3 precipitates from solutions containing its respective ions:[2] The resulting solid decays at a temperature over 20 °C into iron(II) sulfide (FeS) and elemental sulfur:[3] With hydrochloric acid it decays according to the following reaction equation:[4] Greigite, with the chemical formula Fe2+Fe3+2S4, is a mixed valence compound containing both Fe(III) and Fe(II).

It is the sulfur equivalent of the iron oxide magnetite (Fe3O4).

As established by X-ray crystallography, the S anions form a cubic close-packed lattice, and the Fe cations occupy both tetrahedral and octahedral sites.