Metastable phase two-dimensional catalysts provide great flexibility for modifying their chemical, physical, and electronic properties. However, the synthesis of ultrathin metastable phase two-dimensional metallic nanomaterials is highly challenging, mainly due to the anisotropic nature of metallic materials and their thermodynamically unstable ground-state. Here, we report free-standing RhMo nanosheets with atomic thickness and a unique core/shell (metastable phase/stable phase) structure. The polymorphic interface between the core region and shell region stabilizes and activates metastable phase catalysts; the RhMo Nanosheets/C shows excellent hydrogen oxidation activity and stability. Specifically, the mass activities of RhMo Nanosheets/C is 6.96 A mg_Rh⁻¹; this is 21.09 times higher than that of commercial Pt/C (0.33 A mg_Pt⁻¹). Density functional theory calculations suggest that the interface aids in the dissociation of H₂and the H species can then spillover to weak H binding sites for desorption, providing excellent hydrogen oxidation activity for RhMo nanosheets. This work advances the highly controlled synthesis of two-dimensional metastable phase noble metals and provides great directions for the design of high-performance catalysts for fuel cells and beyond.