The effect of point defects on the performance of the Li₃InCl₆ solid electrolyte was investigated using first-principles simulations. The Li-ion conductivity of Li₃InCl₆ was improved by the presence of point defects, such as a 4g site occupation by In or a Cl vacancy. The diffusion of Li ions was activated by the movement of some Li ions to vacant sites in the In layer, and the point defects reduced the energy barrier of this step, resulting in an overall increase in Li-ion diffusivity. The underlying mechanism of this effect was examined in terms of the structural and chemical properties of Li–Cl and In–Cl bonds, which indicated the energy barrier was closely related to changes in the bond distance and covalent interaction between the migrating Li and nearby Cl ions and nonuniformity among In–Cl bonds. Thermodynamic analysis showed that defect formation was unfavorable during conventional high-temperature synthesis; therefore, it is suggested that heating is applied at low temperatures only as a supplementary process to modify the crystallinity after preparing defective Li₃InCl₆ crystals by mechanical or solution-based routes to practically implement the effect of point defects and improve the solid electrolyte performance of Li₃InCl₆.