The development of semicrystalline polymer semiconductors with field-effect mobilities comparable to or even exceeding those of amorphous silicon has exposed limitations of describing charge transport in these materials with disorder-based models developed originally for more disordered, lower mobility polymers. Here, we show that the charge carrier density and temperature dependence of the field-effect electron mobility and Seebeck coefficient in the semicrystalline polymer P(NDI2OD-T2) with varying degrees of crystallinity are incompatible with a description of charge transport being limited by energetic disorder effects. We provide instead direct evidence of low disorder, narrow band conduction. A spatially inhomogeneous density of states and the inclusion of short range electron–electron interactions allow to consistently explain both the measured mobility and Seebeck coefficient. These results provide a rationale for improving thermoelectric efficiency of polymer semiconductors via increasing the extension of the crystalline domains.