The large carrier lifetime mobility product and strong stopping power for high-energy X-rays make halide perovskites an attractive candidate for next-generation X-ray detectors. In particular, high-energy X-rays in the range of several tens of keV require halide perovskite absorber layers with thicknesses exceeding a few millimeters. To avoid carrier scattering caused by grain boundaries at such thicknesses, the utilization of single crystals is desirable. Large-area single crystals are predominantly grown in a freestanding form, and integration onto a substrate is necessary for the fabrication of commercial devices. However, an effective method for integrating large single crystals onto a substrate has not yet been developed. In this study, a large-area (20 cm2) MAPbBr3 single crystal is bonded to an indium tin oxide (ITO) substrate using an ionic liquid, showing strong adhesion strength of 164 kPa. X-ray detectors based on ITO/MAPbBr3 single crystal bonded by methylammonium acetate achieved excellent sensitivity of 91,200 μC Gyair–1 cm–2, the highest among substrate-integrated halide perovskite single crystal X-ray detectors.