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Abstract A thin solid electrolyte with a high Li⁺conductivity is used to separate the metallic lithium anode and the cathode in an all‐solid‐state Li‐metal battery. However, most solid Li‐ion electrolytes have a small electrochemical stability window, large interfacial resistance, and cannot block lithium‐dendrite growth when lithium is plated on charging of the cell. Mg²⁺stabilizes a rhombohedral NASICON‐structured solid electrolyte of the formula Li_1.2Mg_0.1Zr_1.9(PO₄)₃(LMZP). This solid electrolyte has Li‐ion conductivity two orders of magnitude higher at 25 °C than that of the triclinic LiZr₂(PO₄)₃.⁷Li and⁶Li NMR confirm the Li‐ions in two different crystallographic sites of the NASICON framework with 85% of the Li‐ions having a relatively higher mobility than the other 15%. The anode–electrolyte interface is further investigated with symmetric Li/LMZP/Li cell testing, while the cathode–electrolyte interface is explored with an all‐solid‐state Li/LMZP/LiFePO₄cell. The enhanced performance of these cells enabled by the Li_1.2Mg_0.1Zr_1.9(PO₄)₃solid electrolyte is stable upon repeated charge/discharge cycling.