Solid electrolyte containing mobile alkaline ions is a key component in all-solid-state rechargeable batteries. In this work, a relatively new Na⁺-conducting solid electrolyte, Na₃ZnGaS₄, is investigated for a significant increase in ionic conductivity (σ_ion) to a reasonable level. An adjustment in the Zn/Ga ratio results in vacancy formation in Na(2) sites, which leads to a steep increase in σ_ion and to a simultaneous decrease in activation energies (0.32 mS cm⁻¹ and 0.30 eV for hot-pressed Na_2.8Zn_0.8Ga_1.2S₄ at 30 °C). This strategy is ineffective in Na₃ZnGaSe₄, which possesses pre-existing vacancy defects. The preferential formation of vacancies in mobile Na(2), possibility for further improvement of σ_ion, and excellent electrochemical stability are theoretically and experimentally verified. The perfect inertness of Na_2.8Zn_0.8Ga_1.2S₄ in aqueous dispersions (neither decomposition nor hydration) is also confirmed, which suggests that the vacancy-incorporated Na_3−xM(II)_1−xM(III)_1+xS₄ could be a promising class of Na⁺-conducting electrolytes.