Metal halide perovskites have emerged as a new generation of X‐ray detector materials. However, large‐sized MAPbI₃ single crystals (SCs) still exhibit lower performance than MAPbBr₃ SCs in X‐ray detection. DFT (density functional theory) simulations suggest the problem could be overcome by alloying large‐sized cations at the A site. The alloyed process could notably decrease the electron–phonon coupling strength and increase the material defect formation energy. Accordingly, centimeter‐sized alloyed DMAMAPbI₃ (DMA=dimethylammonium) and GAMAPbI₃ (GA=guanidinium) SCs are obtained. Electrical characterizations confirm the GAMAPbI₃ SCs display improved charge collection efficiency. It also exhibits a remarkable reduction of dark current, an important figure of merit for X‐ray detectors. With a judiciously designed device architecture, the overall detector performance confirms GAMAPbI₃ SCs as one of the most sensitive perovskite X‐ray detectors to date.