AbstractRedox‐based memristive devices are one of the most attractive candidates for future nonvolatile memory applications and neuromorphic circuits, and their performance is determined by redox processes and the corresponding oxygen‐ion dynamics. In this regard, brownmillerite SrFeO2.5 has been recently introduced as a novel material platform due to its exceptional oxygen‐ion transport properties for resistive‐switching memory devices. However, the underlying redox processes that give rise to resistive switching remain poorly understood. By using X‐ray absorption spectromicroscopy, it is demonstrated that the reversible redox‐based topotactic phase transition between the insulating brownmillerite phase, SrFeO2.5, and the conductive perovskite phase, SrFeO3, gives rise to the resistive‐switching properties of SrFeOx memristive devices. Furthermore, it is found that the electric‐field‐induced phase transition spreads over a large area in (001) oriented SrFeO2.5 devices, where oxygen vacancy channels are ordered along the in‐plane direction of the device. In contrast, (111)‐grown SrFeO2.5 devices with out‐of‐plane oriented oxygen vacancy channels, reaching from the bottom to the top electrode, show a localized phase transition. These findings provide detailed insight into the resistive‐switching mechanism in SrFeOx‐based memristive devices within the framework of metal–insulator topotactic phase transitions.