Regioregular poly(3-hexylthiophene) was postfunctionalized by a three-step high-yielding reaction producing an electron-accepting 1,1,4,4-tetracyanobuta-1,3-diene unit at the regioregular position of the thiophene rings. Electrochemical measurements of the postfunctionalized polythiophene derivative showed two one-electron reduction waves with the first reduction potential of −0.66 V (vs. Fc/Fc⁺). The electrochemically generated poly(anionic radical)s were confirmed by their electron spin resonance (ESR) signals. The magnetic measurements of the poly(anionic radical)s, produced by chemical reduction with cobaltocene or decamethylcobaltocene, suggested the multiplet state (S ∼ 2/2) at low temperatures.