AbstractThere are enormous yet largely underexplored exotic phenomena and properties emerging from interfaces constructed by diverse types of components that may differ in composition, shape, or crystal structure. It remains poorly understood the unique properties a coherent interface between crystalline and amorphous materials may evoke, and there lacks a general strategy to fabricate such interfaces. It is demonstrated that by topotactic partial oxidation heterostructures composed of coherently registered crystalline and amorphous materials can be constructed. As a proof‐of‐concept study, heterostructures consisting of crystalline P₃N₅ and amorphous P₃N₅O_x can be synthesized by creating amorphous P₃N₅O_x from crystalline P₃N₅ without interrupting the covalent bonding across the coherent interface. The heterostructure is dictated by nanometer‐sized short‐range‐ordered P₃N₅ domains enclosed by amorphous P₃N₅O_x matrix, which entails simultaneously fast charge transfer across the interface and bicomponent synergistic effect in catalysis. Such a P₃N₅/P₃N₅O_x heterostructure attains an optimal adsorption energy for *OOH intermediates and exhibits superior electrocatalytic performance toward H₂O₂ production by adopting a selectivity of 96.68% at 0.4 V_RHE and a production rate of 321.5 mmol h⁻¹ g_catalyst⁻¹ at −0.3 V_RHE. The current study provides new insights into the synthetic strategy, chemical structure, and catalytic property of a sub‐nanometer coherent interface formed between crystalline and amorphous materials.