AbstractSemiconductor‐based X‐ray detectors with low detectable thresholds become critical in medical radiography applications. However, their performance is generally limited by intrinsic defects or unresolved issues of materials, and developing a novel scintillation semiconductor for low‐dose X‐ray detection is a highly urgent objective. Herein, a high‐quality rare‐earth iodate Tm(IO3)3single crystal grown through low‐cost solution processing is reported with a wide bandgap of 4.1 eV and a large atomic number of 53.2. The roles of I—O and Tm—O groups for charge transport in the Tm(IO3)3are revealed with the structural difference between the [101] and[1¯01]crystal orientations. Based on anisotropic responses of material properties and detection performances, it is found that the [1¯01] orientation, the path with fewer I—O groups, achieves a high resistivity of 1.02 × 1011Ω cm. Consequently, a single‐crystal detector exhibits a low dark current and small baseline drifting due to the wide bandgap, high resistivity and less ion migration of Tm(IO3)3, resulting in a low detection limit of 85.2 nGyairs−1. An excellent X‐ray imaging performance with a high sensitivity of 4406.6 µC Gyair−1cm−2is also shown in the Tm(IO3)3device. These findings provide a new material design perspective for high‐performance X‐ray imaging applications.