Exploration of advanced solid electrolytes is a highly relevant research topic for all-solid-state batteries and sensors. One of the effective ways to improve the ion transport is cation substitution. In this work, single-phase Na3P1-x Sb (x) Se-4 polycrystalline compounds are synthesized via solid-state reaction method. The impact of Sb substitution on the chemical structure is revealed by X-ray powder diffraction and Raman spectroscopy. Sb substitution enlarges the unit cell and thus increases the ionic conductivity of Na3P1-x Sb (x) Se-4. Na3SbSe4 as a fully substituted compound achieves the lowest activation energy value of 0.19 eV and the highest ionic conductivity value of 3.7 mS/cm, one of the best values among sulfide solid electrolytes. Together with a low grain-boundary resistance, a single Na+ transference number, and high thermal stability, Na3SbSe4 is a very promising solid electrolyte for all-solid-state sodium batteries.