We report here the first successful demonstration of a "π-conjugated redox polymer" simultaneously featuring a π-conjugated backbone and integrated redox sites, which can be stably and reversibly n-doped to a high doping level of 2.0 with significantly enhanced electronic conductivity. The properties of such a heavily n-dopable polymer, poly{[N,N'-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-T2)), were compared vis-à-vis to those of the corresponding backbone-insulated poly{[N,N'-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5'-[2,2'-(1,2-ethanediyl)bithiophene]} (P(NDI2OD-TET)). When evaluated as a charge storage material for rechargeable Li batteries, P(NDI2OD-T2) delivers 95% of its theoretical capacity at a high rate of 100C (72 s per charge-discharge cycle) under practical measurement conditions as well as 96% capacity retention after 3000 cycles of deep discharge-charge. Electrochemical, impedance, and charge-transport measurements unambiguously demonstrate that the ultrafast electrode kinetics of P(NDI2OD-T2) are attributed to the high electronic conductivity of the polymer in the heavily n-doped state.