A synthetic route was developed to introduce an electron accepting unit, 2,5-di(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP), into the main chain of regioregular poly(3-hexylthiophene) (P3HT) at a low concentration (ca. 4.5 mol % in monomer ratio), in order to lower the optical bandgap and simultaneously retain high hole mobility in P3HT. First, regioregular oligo(3-hexylthiophene) (O3HT), a macromonomer with a narrow polydispersity, was synthesized by Ni-catalyzed Grignard metathesis polymerization and subsequent stannylation of the chain ends. The copolymer was then synthesized by Stifle coupling between the DPP unit and the macromonomer. The copolymer exhibits an extended absorption shoulder up to 750 nm, in addition to the original absorption of P3HT, as a result of intramolecular charge transfer. X-ray diffraction, field-effect transistor measurements, and UV-vis absorption of the polymer films showed that the crystal structure and high hole mobility were retained, owing to the regioregular O3HT blocks. Organic solar cells based on the copolymer and [6,6]-phenyl-C-61-butyric acid methyl ester showed a broad photocurrent response of up to 900 nm and a power conversion efficiency of 2.07% without deterioration of the fill factor.