AbstractThe excellent optoelectronic properties and structural stability of inorganic cesium lead halide perovskites make them promising candidates for multiple types of optoelectronic devices. However, it remains a challenge to fabricate monocrystalline phase‐pure perovskite microstructures by facile low‐temperature solution‐based methods. Herein, a solution‐based method is demonstrated for controlling the crystallization of cesium halide perovskite microstructures. The structure of perovskite crystals is successfully tuned from non‐corner sharing Cs₄PbBr₆ (0D) to corner‐sharing CsPbBr₃ (3D) to layered CsPb₂Br₅ (2D) by controlling water (H₂O) to dimethylsulfoxide (DMSO) ratios. Molecular dynamics simulations and thermodynamic analysis indicate that the relative stability of Pb²⁺ and Br⁻ ions in solution is the key factor in determining which crystals form at different H₂O/DMSO ratios, with Cs⁺ simply incorporated as needed. The phase‐pure 0D crystals exhibit a high photoluminescence quantum yield of 41%, whilst the 2D crystals have an onset of absorption at 350 nm. Furthermore, the as‐synthesized, highly uniform 3D perovskite single crystals are coupled with nanofabricated interdigitated electrodes to show excellent X‐ray detection, with a high sensitivity of 8000 μC Gy_air⁻¹cm⁻² obtained under a 0.5V external bias. This is comparable to many commercial X‐ray detectors (Si, α‐Se) and several times higher than other reported inorganic perovskite materials (CsPbBr₃ quantum dots, Cs₂AgBiBr₆).