We report a route for the synthesis of Na₃SbS₄–Na₂WS₄ superionic conductors from aqueous solution. Na₃SbS₄–Na₂WS₄ has the highest high sodium-ion conductivity (4.28 mS cm⁻¹ at 25 °C) of reported sulfide-based solid electrolytes prepared via liquid-phase methods. The antimony in Na₃SbS₄ is replaced by tungsten in a higher valence state, which is charge-compensated by sodium vacancies in the form Na_3−xSb_1−xW_xS₄. Introducing sodium vacancies while maintaining the 3D conduction pathways increases the pre-exponential factor in the Arrhenius plots of ionic conduction. The development of a facile synthetic protocol, as well as the high ionic conductivity and excellent chemical stability of Na₃SbS₄–Na₂WS₄ in the presence of water, is a major step towards the realization of all-solid-state sodium batteries.