Developing highly efficient oxygen evolution reaction (OER) catalyst for the acidic corrosive operating conditions is a challenging task. Herein, we report the synthesis of uniform RuO₂ clusters with ~ 2 nm in size via electrochemical leaching of Sr from SrRuO₃ ceramic in acid. The RuO₂ clusters exhibit ultrahigh OER activity with overpotential of ~ 160 mV at 10 mA·cm_geo⁻² in 1.0 M HClO₄ solution for 30-h testing. The extended X-ray absorption fine structure measurement reveals enlarged Jahn-Teller distortion of Ru-O octahedra in the RuO₂ clusters compared to its bulk counterpart. Density function theory calculations show that the enhanced Jahn-Teller distortion can improve the intrinsic OER activity of RuO₂.