Compared with thin‐film morphology, 1D perovskite structures such as micro/nanowires with fewer grain boundaries and lower defect density are very suitable for high‐performance photodetectors with higher stability. Although the stability of perovskite microwire‐based photodetectors has been substantially enhanced in comparison with that of photodetectors based on thin‐film morphology, practical applications require further improvements to the stability before implementation. In this study, a template‐assisted method is developed to prepare methylammonium lead bromide (MAPbBr3) micro/nanowire structures, which are encapsulated in situ by a protective hydrophobic molecular layer. The combination of the protective layer, high crystalline quality, and highly ordered microstructures significantly improve the stability of the MAPbBr3 single‐crystal microwire arrays. Consequently, these MAPbBr3 single‐crystal microwire‐array‐based photodetectors exhibit significant long‐term stability, maintaining 96% of the initial photocurrent after 1 year without further encapsulation. The lifetime of such photodetectors is hence approximately four times longer than that of the most stable previously reported perovskite micro/nanowire‐based photodetector; this is thought to be the most stable perovskite photodetector reported thus far. Furthermore, this work should contribute further toward the realization of perovskite 1D structures with long‐term stability. MAPbBr3 single‐crystal microwire arrays with designable shape, controllable size, and high crystalline quality are fabricated. The combination of the hydrophobic molecular protective layer, high crystalline quality, and highly ordered microstructures, improve the intrinsic stability of MAPbBr3 single‐crystal microwire arrays. Thus, a flexible photodetector with long‐term performance stability of more than 1 year is achieved.