Designing high‐performance, precious‐metal‐based, and economic electrocatalysts remains an important challenge in proton exchange membrane (PEM) electrolyzers. Here, a highly active and durable bifunctional electrocatalyst for PEM electrolyzers based on a rattle‐like catalyst comprising a Ni/Ru‐doped Pt core and a Pt/Ni‐doped RuO frame shell, which is topotactically transformed from an icosahedral Pt/Ni/Ru nanocrystal, is reported. The RuO‐based frame shell with its highly reactive surfaces leads to a very high activity for the oxygen evolution reaction (OER) in acidic media, reaching a current density of 10 mA cm at an overpotential of 239 mV, which surpasses those of previously reported catalysts. The Pt dopant in the RuO shell enables a sustained OER activity even after a 2000 cycles of an accelerated durability test. The Pt‐based core catalyzes the hydrogen evolution reaction with an excellent mass activity. A two‐electrode cell employing Pt/RuO as the electrode catalyst demonstrates very high activity and durability, outperforming the previously reported cell performances.