With sp²-nitrogen atoms embedded in an isatin unit, a donor–acceptor (D–A) conjugated polymer AzaBDOPV-2T was developed with a low LUMO level down to −4.37 eV. The lowered LUMO level as well as the conformation-locked planar backbone provide AzaBDOPV-2T with electron mobilities over 3.22 cm² V⁻¹ s⁻¹ tested under ambient conditions, which is among the highest in n-type polymer field-effect transistors (FETs). Our results demonstrate that embedding electron-deficient sp²-nitrogen in conjugated backbones is an effective approach to develop n-type polymer semiconductors with high performance.