AbstractSolid‐state lithium (Li) batteries promise both high energy density and safety while existing solid‐state electrolytes (SSEs) fail to satisfy the rigorous requirements of battery operations. Herein, novel polyoxometalate SSEs, Li₃PW₁₂O₄₀ and Li₃PMo₁₂O₄₀, are synthesized, which exhibit excellent interfacial compatibility with electrodes and chemical stability, overcoming the limitations of conventional SSEs. A high ionic conductivity of 0.89 mS cm⁻¹ and a low activation energy of 0.23 eV are obtained due to the optimized three‐dimensional Li⁺ migration network of Li₃PW₁₂O₄₀. Li₃PW₁₂O₄₀ exhibits a wide window of electrochemical stability that can both accommodate the Li anode and high‐voltage cathodes. As a result, all‐solid‐state Li metal batteries fabricated with Li/Li₃PW₁₂O₄₀/LiNi_0.5Co_0.2Mn_0.3O₂ display a stable cycling up to 100 cycles with a cutoff voltage of 4.35 V and an areal capacity of more than 4 mAh cm⁻², as well as a cost‐competitive SSEs price of $5.68 kg⁻¹. Moreover, Li₃PMo₁₂O₄₀ homologous to Li₃PW₁₂O₄₀ was obtained via isomorphous substitution, which formed a low‐resistance interface with Li₃PW₁₂O₄₀. Applications of Li₃PW₁₂O₄₀ and Li₃PMo₁₂O₄₀ in Li‐air batteries further demonstrate that long cycle life (650 cycles) can be achieved. This strategy provides a facile, low‐cost strategy to construct efficient and scalable solid polyoxometalate electrolytes for high‐energy solid‐state Li metal batteries.