Water oxidation at the anode governs the performance of proton exchange membrane water electrolysis (PEMWE). RuO₂ is a promising alternative to IrO₂, yet its limited durability and unclear reaction/degradation mechanisms hinder practical application. Here, combining electrochemical analysis with advanced characterization, we show that the oxygen evolution reaction (OER) mechanism in RuO₂ depends strongly on polarization current density and history. In fresh, defect-free RuO₂, OER proceeds mainly via the adsorbate evolution mechanism (AEM) at low current density, while the lattice oxygen mechanism (LOM) increases nearly linearly with current density, reaching ~40% at 2 A cm⁻². High-current-density pre-polarization generates lattice defects, promoting LOM even at low current density. These shifts dictate stability: AEM at low currents drives Ru to less active high-valent states, whereas LOM at high current densities depletes lattice oxygen, forms inactive Ru–Ru bonds, and induces surface hydrophobicity, ultimately triggering self-accelerating degradation at the end of long-term operation due to insufficient oxygen replenishment. In contrast, proper AEM–LOM coexistence stabilizes Ru and enhances durability. Guided by these insights, we improve low-current durability via high-current pre-polarization and introduce a cross-flow liquid water supply strategy to both electrodes, enabling stable operation of undoped RuO₂ at 2 A cm⁻² for ~400 h. This work provides new mechanistic insight and practical guidance for durable, low-cost PEMWE.