A crystalline solid electrolyte, Li7P3S11, was synthesized by a liquid-phase reaction of Li2S and P2S5 in an organic solvent. A precursor, which was a mixture of solvated Li3PS4 and Li4P2S7, was prepared by mixing Li2S and P2S5 powders in 1,2-dimethoxyethane (DME) solvent. After a vacuum drying of the precursor, the crystalline phase of Li7P3S11 was obtained by heat treatment at 250 °C for 1 h in Ar atmosphere. The Li7P3S11 sample showed high ionic conductivity of 2.7 × 10−4 S cm−1 at room temperature. The liquid-phase synthesis of the solid electrolyte has advantages for mass-production of all-solid-state batteries.