Recently, in all solid state batteries, the sulfide based solid electrolytes have received increasing attention due to their high ionic conductivity, good mechanical features and better chemical stability. Therefore, in the present study, we have synthesized sodium superionic conducting novel sulfide based inorganic solid electrolyte (Na11Sn2AsS12) by solid state reaction method. The prepared solid electrolyte (Na11Sn2AsS12) has been characterized by different techniques such as XRD, SEM, XPS, TGA, EIS and LSV to study its various properties such as structure, surface morphology, thermal stability, dielectric properties, ionic conductivity and electrochemical stability window for sodium ion battery applications. The XRD analysis confirms the coexisting two phases- tetragonal and cubic with the phase fraction of 0.69 and 0.31 respectively. The SEM study reveals the irregular shape and dense morphology of solid electrolyte. On the other hand, TGA analysis shows that the prepared solid electrolyte is suitable for high temperature battery application. The ionic and transport studies confirm that the synthesized Na11Sn2AsS12 is purely ionic in nature having ionic conductivity found to be 1.14×〖10〗^(-4) S/cm and negligible electronic conductivity ~ 1.43×〖10〗^(-10) S/cm at room temperature. Furthermore, the detailed ionic conduction mechanism is studied using temperature and frequency dependent ac impedance analysis. In addition, the synthesized solid electrolyte Na11Sn2AsS12 exhibits wide electrochemical window (~7.0 V) and high diffusion coefficient (1.3×〖10〗^(-7) cm2/s) showing suitable electrolyte property for solid state sodium ion battery application.