The synthesis of a novel naphthalenediimide (NDI)-bithiazole (Tz2)-based polymer [P(NDI2OD-Tz2)] is reported, and structural, thin-film morphological, as well as charge transport and thermoelectric properties are compared to the parent and widely investigated NDI-bithiophene (T2) polymer [P(NDI2OD-T2)]. Since the steric repulsions in Tz2 are far lower than in T2, P(NDI2OD-Tz2) exhibits a more planar and rigid backbone, enhancing π–π chain stacking and intermolecular interactions. In addition, the electron-deficient nature of Tz2 enhances the polymer electron affinity, thus reducing the polymer donor–acceptor character. When n-doped with amines, P(NDI2OD-Tz2) achieves electrical conductivity (≈0.1 S cm−1) and a power factor (1.5 µW m−1 K−2) far greater than those of P(NDI2OD-T2) (0.003 S cm−1 and 0.012 µW m−1 K−2, respectively). These results demonstrate that planarized NDI-based polymers with reduced donor–acceptor character can achieve substantial electrical conductivity and thermoelectric response.