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To achieve high‐energy‐density and safe lithium‐metal batteries (LMBs), solid‐state electrolytes (SSEs) that exhibit fast Li‐ion conductivity and good stability against lithium metal are of great importance. This study presents a systematic exploration of selenide‐based materials as potential SSE candidates. Initially, Li8SeN2 and Li7PSe6 were selected from 25 ternary selenides based on their ability to form stable interfaces with lithium metal. Subsequently, their favorable electronic insulation and mechanical properties were verified. Furthermore, extensive theoretical investigations were conducted to elucidate the fundamental mechanisms underlying Li‐ion migration in Li8SeN2, Li7PSe6, and derived Li6PSe5X (X = Cl, Br, I). Notably, the highly favorable Li‐ion conduction mechanism of vacancy diffusion was identified in Li6PSe5Cl and Li7PSe6, which exhibited remarkably low activation energies of 0.21 and 0.23 eV, and conductivity values of 3.85 × 10−2 and 2.47 × 10−2 S cm−1 at 300 K, respectively. In contrast, Li‐ion migration in Li8SeN2 was found to occur via a substitution mechanism with a significant diffusion energy barrier, resulting in a high activation energy and low Li‐ion conductivity of 0.54 eV and 3.6 × 10−6 S cm−1, respectively. Throughout this study, it was found that the ab initio molecular dynamics and nudged elastic band methods are complementary in revealing the Li‐ion conduction mechanisms. Utilizing both methods proved to be efficient, as relying on only one of them would be insufficient. The discoveries made and methodology presented in this work lay a solid foundation and provide valuable insights for future research on SSEs for LMBs. [ABSTRACT FROM AUTHOR]