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 Cs2AgBiBr6 double perovskite single crystals (SCs) is reported. Two types of Cs2AgBiBr6 SCs, 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 Cs2AgBiBr6 SCs (2.65 × 109 cm−3) is one order of magnitude lower than that in Bi-normal Cs2AgBiBr6 SCs (3.85 × 1010 cm−3). Using laser-induced photocurrent measurements, the obtained mobility–lifetime (μ–τ) product in Bi-poor Cs2AgBiBr6 SCs is 1.47 × 10−3 cm2 V−1, indicating their great potentials for gamma-ray detection. Further, the fabricated detector based on Bi-poor Cs2AgBiBr6 SC shows response to 59.5 keV gamma-ray with an energy resolution of 13.91%.