Provided is a method of producing a sulfide solid electrolyte with which the capacity retention of an all-solid-state battery can be improved. The method of producing a sulfide solid electrolyte comprises synthesizing material for a sulfide solid electrolyte from raw material for an electrolyte; and after said synthesizing, heating the material for a sulfide solid electrolyte in a flow of a gas at a temperature of no less than a melting point of elemental sulfur, the gas being able to form a chemical bond with the elemental sulfur.