A novel electrochromic material, poly(1H-benzo[g]indole) (PBIn), was synthesized electrochemically by direct anodic oxidation of 1H-benzo[g]indole (BIn) in CH₃CN. The electrochemical and spectroelectrochemical characterization of this novel polyindole derivative was reported. Spectroelectrochemical studies show that PBIn can change from tan (neutral) to pale green (oxidized) along with the increasing potentials. An ECD based on PBIn and poly(3,4-ethylenedioxythiophene) (PEDOT) was also fabricated and showed a good electrochromic performance. The ECD constructed by PBIn and PEDOT has good optical contrast (41% at 610 nm and 50% at 1320 nm), high coloration efficiency (720 cm² C⁻¹ at 610 nm and 590 cm² C⁻¹ at 1320 nm), fast response time (0.6 s at 610 nm), better optical memory and long-term stability. A clear change from green (neutral) to navy blue (oxidized) of the ECD is demonstrated with a robust cycle life.