AbstractThe development of solid‐state electrolytes (SSEs) with outstanding comprehensive performance is currently a critical challenge for achieving high energy density and safer solid‐state batteries (SSBs). In this study, a strategy of nano‐confined in situ solidification is proposed to create a novel category of molten guest‐mediated metal–organic frameworks, named MGM–MOFs. By embedding the newly developed molten crystalline organic electrolyte (ML₂₀) into the nanocages of anionic MOF–OH, MGM–MOF–OH, characterized by multi‐modal supramolecular interaction sites and continuous negative electrostatic environments within nano‐channels, is achieved. These nanochannels promote ion transport through the successive hopping of Li⁺ between neighbored negative electrostatic environments and suppress anion movement through the chemical constraint of the hydroxyl‐functionalized pore wall. This results in remarkable Li⁺ conductivity of 7.1 × 10⁻⁴ S cm⁻¹ and high Li⁺ transference number of 0.81. Leveraging these advantages, the SSBs assembled with MGM–MOF–OH exhibit impressive cycle stability and a high specific energy density of 410.5 Wh kg_{anode + cathode + electrolyte}⁻¹ under constrained conditions and various working temperatures. Unlike flammable traditional MOFs, MGM–MOF–OH demonstrates high robustness under various harsh conditions, including ignition, high voltage, and extended to humidity.