A solid electrolyte contains a compound that has a crystal phase having an argyrodite-type crystal structure and that is represented by LiaPSbXc, where X is at least one elemental halogen, a represents a number of 3.0 or more and 6.0 or less, b represents a number of 3.5 or more and 4.8 or less, and c represents a number of 0.1 or more and 3.0 or less. The proportion of the crystal phase with an argyrodite-type structure relative to all crystal phases constituting the solid electrolyte is 97.0 wt % or more. The compound has a lattice strain of less than 0.10%. The solid electrolyte preferably exhibits a lithium ion conductivity of 4.0 mS/cm or more.