• A poly(diallyl dimethyl ammonium chloride) (PDDAC) is proposed as an new electrolyte for chloride ion batteries. • The chloride ion conductivity is significantly varied by PEG plasticizer. • Chloride ion conductivity of 2.63 × 10 −5 S cm −1 is obtained at 303 K. • The chloride ion conducting capability is evidenced in a chloride ion battery. Chloride-ion batteries (CIBs) based on shuttling of chloride ion have recently attracted attention because of the safety, abundant resources, and high theoretical energy density. Herein, a polymer of poly(diallyl dimethyl ammonium chloride) (PDDAC) plasticized by poly(ethylene glycol) (PEG) is proposed as an new electrolyte for chloride ion batteries. The ionic conductivity of polymer electrolyte is significantly varied by the amount of plasticizer. A flexible, transparent polymer film with 70 wt% of PEG (Mw = 300) shows the highest chloride ion conductivity (2.63 × 10 −5 S cm −1 ) at 303 K. The chloride ion conducting capability is also evidenced in a lithium free chloride ion battery with CuCl as a cathode and Mg as an anode.