Metal halide perovskite single crystals (SCs) have emerged as promising candidates for high‐performance X‐ray detectors. However, mitigating the adverse effects of thermal and interfacial stress during SC growth remains a significant challenge. In this study, the solution growth of high‐quality perovskite SCs using silicone containers through a constant‐temperature autonomous crystallization process is presented. Unlike conventional hard glass vessels, soft silicone containers significantly reduce the negative impact of thermal and interfacial stress on SC growth. Additionally, the microporous nature of silicone containers permits solvent leakage, facilitating autonomous SC growth at constant temperatures. The hydrophilic surface of the silicone further increases the interfacial nucleation barrier and enhances mass transfer, promoting the rapid growth of larger SCs. This multifaceted approach results in MAPbBr3 SCs with an exceptionally low defect density of 2.15×108 cm−3 and an optimal full‐width‐at‐half‐maximum of X‐ray diffraction rocking curves at 19.04 arcsec, consequently leading to an ultralow X‐ray detection limit of 850 pGyair s−1 for the X‐ray detectors. The superior quality of the SCs, combined with a low‐temperature flip‐chip bonding process, enables the integration of the crystals with pixelated arrays on a printed circuit board for both single‐photon detection and low‐dose X‐ray imaging.