Donor–acceptor-based poly(E)-4-(3,4′-didodecyl-5′-(2-(3-dodecylthiophen-2-yl)vinyl)-2,2′-bithiophen-5-yl)-7-(4-dodecylthiophen-2-yl)benzo[c][1,2,5]selenadiazole (11) has been synthesized by a Stille coupling reaction. This polymer has a low energy band gap between the HOMO and LUMO energy levels of 1.75 eV. The polymer exhibited good thermal stability. An OTFT prepared using this polymer displayed high hole mobility of 0.097 cm² V⁻¹ s⁻¹ at 200 °C, a high on/off ratio of 7.8 × 10⁴, and a low threshold voltage of 11.2 V. When compared with as-cast films, annealed films exhibited higher mobility, which was attributed to an increase in crystallinity with an increase in the annealing temperature.