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Abstract Lead halide perovskites have recently attracted intensive attention as competitive alternative candidates of legacy compound materials CdTe, CdZnTe, and TlBr for high sensitivity energy‐resolving gamma‐ray detection at room temperature. However, the use of lead in these lead halide perovskites, which is necessary for increasing the stopping power of gamma radiation, poses a serious environmental concern due to the high toxicity of lead. In this regard, environmental‐friendly perovskite‐based gamma‐ray detector materials with key energy‐resolving capabilities are highly desired. Here, the gamma energy‐resolving performance of a new class of all‐inorganic and lead‐free Cs₂AgBiBr₆double perovskite single crystals (SCs) is reported. Two types of Cs₂AgBiBr₆SCs, prepared by Bi‐normal and Bi‐poor precursor solutions, respectively, have been grown. Their mobilities and response to gamma radiation are presented. Density of trap states in Bi‐poor Cs₂AgBiBr₆SCs (2.65 × 10⁹ cm⁻³) is one order of magnitude lower than that in Bi‐normal Cs₂AgBiBr₆SCs (3.85 × 10¹⁰ cm⁻³). Using laser‐induced photocurrent measurements, the obtained mobility–lifetime (μ–τ) product in Bi‐poor Cs₂AgBiBr₆SCs is 1.47 × 10⁻³ cm² V⁻¹, indicating their great potentials for gamma‐ray detection. Further, the fabricated detector based on Bi‐poor Cs₂AgBiBr₆SC shows response to 59.5 keV gamma‐ray with an energy resolution of 13.91%.