Solid electrolytes have attracted considerable interest in rechargeable batteries because of their potential high safety, inhibition of electrode dissolution, and large electrochemical window. However, their development in some new battery concepts such as room-temperature halide ion batteries has been scarce. Herein, we develop the inorganic halide perovskite of CsSnCl3 prepared by mechanical milling and subsequent mild heat treatment as the potential solid electrolyte for chloride ion batteries (CIB). Benefiting from its high structural stability against a phase transformation to monoclinic structure at room temperature, the as-prepared cubic CsSnCl3 achieves an impressive electrochemical performance with the highest ionic conductivity of 3.6 × 10–4 S cm–1 and a large electrochemical window of about 6.1 V at 298 K. These values are much higher than 1.2 × 10–5 S cm–1 and 4.25 V of the previously reported solid polymer electrolyte for CIBs. Importantly, the chloride ion transfer of the as-prepared CsSnCl3 electrolyte is demonstrated by employing the electrode couples of SnCl2/Sn and BiCl3/Bi.