LAUSR

2D perovskites are widely recognized as promising candidates for X‐ray detection. However, most 2D perovskites are organic‐inorganic hybrids with low equivalent atomic numbers, limiting their X‐ray absorption capacity, while all‐inorganic 2D perovskites typically exhibit <110> orientation, suffering from severe structural distortion and poor carrier transport. To address these challenges, an all‐inorganic sulfur‐halide mixed perovskite, BiCuSCl2, is proposed featuring a 2D <100>‐oriented structure, and further enhance its performance via BiOCl heteroepitaxial passivation. The all‐inorganic composition endows BiCuSCl2 with a high equivalent atomic number across the entire X‐ray energy range, while the <100> orientation minimizes structural distortion. Meanwhile, the epitaxial BiOCl layer effectively passivates grain boundary defects and suppresses ionic migration. As a result, the BiCuSCl2 wafer detector achieves a significantly improved µτ product (from 1.98 × 10−3 to 7.76 × 10−3 cm2 V−1) and high resistivity (1.12 × 1011 Ω cm). The optimized device exhibits an ultralow dark current density (6.32 × 10−11 A cm−2) and a sensitivity of 219 µC Gy−1 cm−2 under identical bias. The realized specific detectivity (ratio of sensitivity to noise current density, 3.47 × 1012 µC Gy−1 A−1) is among the highest all lead‐free perovskite X‐ray detectors.