The structures of the red polymorph (space group P 1 ) and the black polymorph (space group P 2 1 / c ) of 4,4′-dimethyl-6,6′-dichlorothioindigo were solved from single-crystal samples. For both polymorphs, the spatial stacking of the flat molecules is driven by π-stacking and noncovalent interactions within the layers. Spectroscopic (UV–vis, IR and photoluminescence) and thermal properties of the red polymorph were investigated experimentally. The bandgap of this polymorph was estimated as 2.08 eV at room temperature. It is demonstrated that the electric conductivity of the red polymorph follows the hopping mechanism.