Alloying platinum (Pt) with transition metals (M), as an established class of electrocatalysts, reduces the use of Pt and improves the electrocatalytic performance. However, the stability of transition metals in nanostructured platinum alloys is a fundamental and practical problem in electrocatalysis, due to leaching of transition metals under acidic operating condition. Here, a corrosion method has been developed for a Pt−Cu electrocatalyst with high activity (6.6 times that of commercial Pt/C) and excellent stability for the methanol oxidation reaction (MOR) under acidic operating conditions. The mechanism of formation has been studied, and possible mesostructured re‐formation and atomic re‐organization have been proposed. This work offers an effective strategy for the facile synthesis of a highly acid‐stable PtM alloying and opens a door to high‐performance design for electrocatalysts. Acid tested: An ultimate corrosion method to Pt−Cu electrocatalysts is presented that greatly promotes electrocatalytic activity and stability for methanol oxidation under acidic conditions. The formation mechanism has been investigated, and a mesostructured re‐formation and atomic re‐organization process is proposed. Our method could potentially be extended to the facile synthesis of highly acid stable Pt‐based alloys with excellent electrocatalytic performance.