Thioindigo is an organosulfur compound that is used to dye polyester fabric.

A synthetic dye, thioindigo is related to the plant-derived dye indigo, replacing two NH groups with two sulfur atoms to create a shade of pink.

Thioindigo is generated by the alkylation of the sulfur in thiosalicylic acid with chloroacetic acid.

The resulting thioether cyclizes to 2-hydroxythianaphthene, which is easily converted to thioindigo.

[1] The related compound 4,7,4',7'-tetrachlorothioindigo, also a commercially important dye (Pigment Red 88), can be prepared by chlorination of thioindigo.