Developing acid-stable manganese-based catalysts for the oxygen evolution reaction (OER) is pivotal for advancing proton exchange membrane water electrolysis (PEMWE). Here, we present a selenium-doped MnO₂ catalyst, where the synergistic effects of Se and oxygen defects stabilize Mn³⁺ species and regulate *OH adsorption dynamics. In situ spectroscopic studies and density functional theory (DFT) calculations confirm that Se doping modulates the electronic structure of Mn centers, lowering the energy barrier for *OH deprotonation and accelerating OER kinetics. In 0.5 M H₂SO₄, Se-MnO₂ achieves current densities of 10 and 100 mA·cm⁻² with overpotentials of 345 ± 5 and 398 ± 5 mV, respectively, outperforming commercial RuO₂. Integrated into PEM electrolyzers, the catalyst demonstrates exceptional stability over 400 h under dynamic current densities (100–500 mA·cm⁻²), showcasing structural integrity and negligible activity decay. The strategic doping of selenium significantly enhances catalytic performance, thereby offering a promising pathway toward the development of cost-effective electrocatalysts for applications under acidic conditions.