Organic–inorganic hybrid perovskite CH₃NH₃PbI₃ (MAPbI₃) has been widely investigated in near-infrared (NIR) photodetection and X-ray detection due to its superior optoelectronic properties. However, the preparation of high-quality MAPbI₃ crystals remains a great challenge, which severely limits their potential applications. In this work, we use the small-sized MAPbI₃ single crystals as raw materials, and prepare the high-quality MAPbI₃ polycrystalline wafers by a hot-pressing method. The MAPbI₃ polycrystalline wafers are applied in both NIR photodetectors and X-ray detectors, which achieve high performances. The NIR photodetector can achieve the responsivity of 2.1 A W⁻¹, the external quantum efficiency of 332.36%, and the detectivity of 1.5 × 10¹² Jones in the wavelength of 785 nm at 5 V. The charge carrier mobility and lifetime product of the MAPbI₃ polycrystalline wafers is determined to be 7.2 × 10⁻⁴ cm² V⁻¹, and the sensitivity of the X-ray detector can reach 3.1 × 10⁵ μC Gy⁻¹ cm⁻² at 5 V and at the dose rate of regular medical diagnostics (5.5 μGy s⁻¹). Most importantly, NIR bioimaging and medical X-ray imaging are successfully realized with the above NIR photodetector and X-ray detector, respectively. The results reported in this work show that MAPbI₃ polycrystalline wafers can be regarded as competitive candidates for NIR bioimaging and medical X-ray imaging.