Halide double perovskites have recently bloomed as the green candidates for optoelectronic applications, such as X-ray detection. Despite great efforts, the exploration of promising organic-inorganic hybrid double perovskites toward X-ray detection remains a blank. Here, for the first time, we present single crystals of the lead-free hybrid double perovskite, (BA) 2 CsAgBiBr 7 (BA + is n-butylammonium), featuring the unique 2D multilayered quantum-confined motif, which enable quite large μτ (mobility-lifetime) product up to ~1.21 × 10 -3 cm 2 V -1 . This figure-of-merit realized in 2D hybrid double perovskites is unprecedented and comparable with that of CH 3 NH 3 PbI 3 wafers. Further, (BA) 2 CsAgBiBr 7 crystals also exhibit other intriguing attributes for X-ray detection, including high bulk resistivity, low density of defects and traps, and large X-ray attenuation coefficient. Consequently, vertical-structure crystal device under X-ray source yields a superior sensitivity of 4.2 μC Gy air -1 cm -2 . This work suggests great potentials of 2D hybrid double perovskites as candidate toward X-ray detection, and knocks the door for their future optoelectronic device applications.