Oxygen stoichiometry plays a crucial role in determining the crystalline structure and physical properties of transition metal oxides (TMOs). Tuning the oxygen content via an electrochemical redox reaction can effectively manipulate the functionalities of TMOs, which is harnessed in many cutting-edge energy and information technologies such as fuel cells, [1] rechargeable batteries, [2] supercapacitors, [3] and memory devices. [4] The redox reaction in certain TMOs can be enabled by a so-called topotactic phase transformation, manifesting itself as the insertion/release of a large amount of oxygen ions without breaking the lattice framework. For example, for an ABO 3 perovskite (PV) phase, upon forming ordered oxygen vacancy channels in its lattice, it transforms into an ABO 2.5 brownmillerite (BM) phase. Along with the structural change, many intriguing physical phenomena emerge owing to the couplings between lattice, charge, and Resistive switching (RS) memory has stayed at the forefront of next-generation nonvolatile memory technologies. Recently, a novel class of transition metal oxides (TMOs), which exhibit reversible topotactic phase transformation between insulating brownmillerite (BM) phase and conducting perovskite (PV) phase, has emerged as promising candidate materials for RS memories. Nevertheless, the microscopic mechanism of RS in these TMOs is still unclear. Furthermore, RS devices with simultaneously high density and superior memory performance are yet to be reported. Here, using SrFeO x as a model system, it is directly observed that PV SrFeO 3 nanofilaments are formed and extend almost through the BM SrFeO 2.5 matrix in the ON state and are ruptured in the OFF state, unambiguously revealing a filamentary RS mechanism. The nanofilaments are ≈10 nm in diameter, enabling to downscale Au/ SrFeO x /SrRuO 3 RS devices to the 100 nm range for the first time. These nanodevices exhibit good performance including ON/OFF ratio as high as ≈10 4 , retention time over 10 5 s, and endurance up to 10 7 cycles. This study significantly advances the understanding of the RS mechanism in TMOs exhibiting topotactic phase transformation, and it also demonstrates the potential of these materials for use in high-density RS memories.