C-H functionalization of pi-conjugated polymers is a straightforward approach to changing their optoelectronic properties and aggregation behavior; however, the functionalization methodologies and applicable precursor polymers remain limited. Here, we demonstrated the electrochemical aromatic C-H chlorination of pi-conjugated polymers using aluminum chloride (AlCl3). AlCl3 facilitated the anodic oxidation of precursor polymers in acetonitrile (MeCN), and the AlCl3/MeCN system exhibited greater oxidation tolerance than other electrolyte systems. Given these discoveries, we achieved anodic chlorination of poly(3-hexylthiophene) with high current efficiency and expanded applicable precursor polymers such as poly(p-phenylene) derivatives and poly(9,9-dioctylfluorene) (PFO). The regioselectivity of the chlorination of PFO was clarified on the basis of detailed nuclear magnetic resonance studies of the chlorinated PFO. The optoelectronic properties and aggregation behavior of the precursor polymers were substantially changed after chlorination.