A method of preparing a sulfide solid electrolyte, the method including: first contacting a starting materials including Li2S, P2S5, and LiI in a first solvent to provide a precursor; and second contacting the precursor with a second solvent to prepare the sulfide solid electrolyte, wherein the first solvent includes a C1-C3 alkyl group or a cyclic ether compound which is unsubstituted or substituted with a C1-C3 alkoxy group, and the second solvent includes a C1-C10 hydrocarbon substituted with a C1 to C6 alkoxy group.