AbstractDeveloping non-noble catalysts with superior activity and durability for oxygen evolution reaction (OER) in acidic media is paramount for hydrogen production from water. Still, challenges remain due to the inadequate activity and stability of the OER catalyst. Here, we report a cost-effective and stable manganese oxybromide (Mn_7.5O₁₀Br₃) catalyst exhibiting an excellent OER activity in acidic electrolytes, with an overpotential of as low as 295 ± 5 mV at a current density of 10 mA cm⁻². Mn_7.5O₁₀Br₃ maintains good stability under operating conditions for at least 500 h. In situ Raman spectroscopy, X ray absorption near edge spectroscopy, and density functional theory calculations confirm that a self-oxidized surface with enhanced electronic transmission capacity forms on Mn_7.5O₁₀Br₃ and is responsible for both the high catalytic activity and long-term stability during catalysis. The development of Mn_7.5O₁₀Br₃ as an OER catalyst provides crucial insights into the design of non-noble metal electrocatalysts for water oxidation.