ABSTRACT The rational design of acid‐stable, iridium‐free electrocatalysts for the oxygen evolution reaction (OER) is critical for advancing proton exchange membrane water electrolysis (PEMWE), yet balancing activity and durability remains a formidable challenge. Herein, we report a RuO 2 /Mn 3 O 4 heterojunction with engineered oxygen vacancies (O v ) as a durable, high‐performance iridium alternative. Engineering triggers substantial electron transfer from Mn 3 O 4 to RuO 2 , lowering the average Ru oxidation state from +3.69 to +3.34 and increasing d ‐band occupancy from 4.23 to 4.50. This enhanced occupancy strengthens Ru–O covalency via intensified coupling with O 2 p orbitals, corroborated by density functional theory calculations showing a reduced energy barrier of potential‐determining step by 0.62 eV. In situ spectroscopy further reveals a distinctive dual H 2 O adsorption configuration at adjacent Ru–O v sites, enabling direct O–O coupling and promoting a more efficient OER pathway. Consequently, the optimized RuO 2 /Mn 3 O 4 ‐O v catalyst achieves an exceptionally low overpotential of 185 mV at 10 mA cm − 2 and a turnover frequency of 4.33 s − 1 at 185 mV—188‐fold higher than commercial RuO 2 . Notably, it maintains stable operation for over 200 h at 100 mA cm − 2 in 0.1 M HClO 4 , highlighting its promise for replacing iridium catalysts in PEMWE applications.