<p>Designing highly efficient acidic oxygen evolution reaction (OER) electrocatalysts that combine high activity, long-term stability, and low cost has become crucial for proton exchange membrane water electrolyzers. RuO₂ catalysts, following a kinetically favorable lattice oxygen mechanism, exhibit superior intrinsic activity but suffer from inferior stability for acidic OER. Current research predominantly focuses on sacrificing the intrinsic activity of RuO₂ to enhance stability. The activity-stability trade-off in RuO₂-based catalysts remains a central research focus. Here, we propose a grain boundary engineering strategy to boost the OER activity and stability of RuO₂ in acidic media. Grain boundaries in RuO₂ catalysts do not alter the Ru valence state and Ru–O covalency, thus enabling grain boundary-rich RuO₂ catalysts (GB-RuO₂) to follow the lattice oxygen mechanism pathway. We discover that grain boundaries can increase the content of coordinatively unsaturated Ru⁶⁺ species and promote the deprotonation kinetics of adsorbed oxygen species, thus balancing acidic OER activity and stability. The optimal GB-RuO₂ catalysts achieve an exceptionally low acidic OER overpotential of 159 mV at 10 mA cm⁻² and a minor potential increase of 44 mV after 24 h at 50 mA cm⁻², significantly surpassing RuO₂ catalysts with a low grain boundary.</p>