Solid inorganic electrolytes (SIEs) are promising materials for ion exchange membranes used in reverse electrodialysis (RED) processes to produce clean energy. However, SIEs have not yet been used in RED. Herein, Mg-substituted Na3Zr2Si2PO12 (NZMSP), an inorganic sodium superionic conductor, is used for the first time as a single-ion conducting ion exchange membrane in RED. The bilayer freestanding NZMSP membranes feature a sub-10-μm dense layer, delivering ultrahigh permselectivity (nearing 100%) and low resistance, along with improved power density compared to commercial cation exchange membranes. This work reports a new method to prepare high-performance SIE-based membranes. The enhanced power generation and permselectivity using this SIE membrane present a milestone in the field and will inspire further investigations.