Systematic creation of polymeric semiconductors from novel building blocks is critical for improving charge transport properties in organic field-effect transistors (OFETs). A series of ultralow-bandgap polymers containing thienoisoindigo (TIIG) as a thiophene analogue of isoindigo (IIG) is synthesized. The UV-Vis absorptions of the TIIG-based polymers (PTIIG-T, PTIIG-Se, and PTIIG-DT) exhibit broad bands covering the visible to near-infrared range of up to 1600 nm. All the polymers exhibit unipolar p-channel operations with regard to gold contacts. PTIIG-DT with centrosymmetric donor exhibits a maximum mobility of 0.20 cm2 V−1 s−1 under gold contacts, which is higher than those of the other polymers containing axisymmetric donors. Intriguingly, OFETs fabricated with aluminum electrodes show ambipolar charge transport with hole and electron mobilities of up to 0.28 (PTIIG-DT) and 0.03 (PTIIG-T) cm2 V−1 s−1, respectively. This is a record value for the hitherto reported TIIG-based OFETs. The finding demonstrates that TIIG-based polymers can potentially function as either unipolar or ambipolar semiconductors without reliance on the degree of electron affinity of the co-monomers.