Developing electrocatalysts with ultrathin nanostructures and high mesoporosity is a relevant high‐priority research direction toward enhancing the performance of noble metals. Herein, mesoporous, highly excavated octahedral PtCu3 nanostructures are prepared by a facile one‐pot synthesis. The mesoporous, highly excavated octahedral PtCu3 nanostructures are built with mutually perpendicular interlaced mesoporous nanosheets with a thickness of ≈4.5 nm. Benefiting from its mesoporous features, three‐dimensional (3D) open surfaces, ultrathin nanosheets, and a Cu‐rich surface, PtCu3 exhibits excellent electrocatalytic performance and high antipoisoning activity toward the methanol oxidation reaction.