Solid-state batteries with nonflammable inorganic solid electrolytes provide a fundamental solution for resolving safety concerns. Li₇La₃Zr₂O₁₂ (LLZO) has been considered as a promising candidate for solid electrolytes, due to its high Li⁺ conductivity and chemical/electrochemical compatibility with Li metal. However, LLZO electrolytes are known to react with H₂O and CO₂ to form lithium carbonates (Li₂CO₃) on the surface when exposed to the ambient air, resulting in the significant degradation in Li⁺-conduction properties. Herein, we propose an effective approach for improving the air-stability of LLZO via hydrophobic polymer encapsulation. For encapsulation of LLZO powders, polyurethane-based polymers are designed and synthesized to have high hydrophobicity and ionic conductivity by engineering soft segment and hydrophobic chain extender. Bare LLZO and polymer-encapsulated LLZO (P-LLZO) powders are subjected to accelerated durability tests (ADTs) in which the concentrations of O₂, H₂O, and CO₂ are precisely controlled to promote the interfacial reactions. Surface characterization studies reveal that the polymer encapsulation of LLZO effectively mitigates the interfacial degradation (Li₂CO₃ formation) by preventing the direct contact between LLZO and H₂O/CO₂. Furthermore, a biphasic solid electrolyte (BSE) fabricated using ADT-tested P-LLZO powders exhibits higher ionic conductivity as compared with that of BSE with ADT-tested LLZO, proving the efficacy of the polymer encapsulation. The findings of this study would be essential in understanding the role of interfacial engineering in mitigating the degradation of Li⁺-conduction properties and developing highly conductive and stable LLZO solid electrolytes.