The development of efficient oxygen evolution reaction (OER) electrocatalysts is of vital importance for acidic water decomposition. Currently, the rutile structured RuO₂ is considered to be the best candidate and has been studied widely. However, the state-of-the-art rutile RuO₂ still suffers from unsatisfactory activity and stability, while other crystalline structures of Ru-based oxides are seldom reported as OER electrocatalysts. The present work, for the first time, successfully synthesizes orthorhombic (Ru, Mn)₂O₃ electrocatalyst through cation exchange. The orthorhombic (Ru, Mn)₂O₃ particles exhibit the outstanding electrocatalysis performance as OER electrocatalyst, showing an ultralow overpotential of 168 mV at 10 mA cm⁻² in acidic water and good stability in 40 h of OER. The outstanding electrocatalysis performance is attributed to the distinct Ru d-band structure together with the larger surface density of Ru active site, which reduces the free energy of the rate-limiting step during OER.