Thiirene is an organosulfur compound with the formula C<sub>2</sub>H<sub>2</sub>S. It can be viewed as a derivative of cyclopropene, but with the methylene group replaced by sulfur. It is antiaromatic and very labile.
No thiirene has been isolated at room temperature, but they have been observed spectroscopically at low temperatures.
Thiirene-S-oxides and S-alkylthiirenium salts have been characterized by X-ray crystallography.