Terthiophene is the organic compound with the formula [C<sub>4</sub>H<sub>3</sub>S]<sub>2</sub>C<sub>4</sub>H<sub>2</sub>S. It is an oligomer of the heterocycle thiophene, a shorter oligomer is dithienyl, and the parent polymer is polythiophene. In the most common isomer of terthiophene, two thienyl groups are connected via their 2 positions to a central thiophene, also at the carbon atoms flanking the sulfur.
Terthiophene is prepared by the nickel- or palladium-catalysed coupling reaction of 2,5-dibromothiophene with the Grignard reagent derived from 2-bromothiophene.
This substance is likely responsible for the insecticidal activity of Tagetes minuta as it can react with light and oxygen to make singlet oxygen.
Together with derivatives of 2,2'-bithiophene, various chlorinated terthiophenes occur naturally in thistles.
Terthiophene has been employed as building block for the organic semi-conductor polythiophene.