The development of highly active electrocatalysts for the oxygen evolution reaction (OER) in proton exchange membrane water electrolyzers (PEMWEs) has garnered significant scientific intrigue. Herein, we report a facile two-step hydrothermal and annealing method for synthesizing cerium-doped ruthenium dioxide nanoparticles (Ce-RuO₂) as efficient OER electrocatalyst in PEM electrolyzer, which exhibits superior OER catalytic performance and durability. More impressively, in-situ characterizations of Ce-RuO₂ as the anode catalyst in membrane electrode assembly is employed, and the results demonstrate that it exhibits reduced kinetic losses, higher catalytic activity, and enhanced stability compared to commercial RuO₂ in PEMWE. In addition, density functional theory (DFT) calculations further confirm that Ce doping maintains its valence state and redistributing the density states of Ru species, thereby enhancing its activity and stability during the OER processes.