AbstractThe unsatisfactory oxygen evolution reaction (OER) activity of IrO₂ has intensively raised the cost and energy consumption of hydrogen generation from proton exchange membrane water electrolyzers. Here, the acidic OER activity of the rutile IrO₂ is significantly enhanced by the incorporation of trivalent metals (e.g., Gd, Nd, and Pr) to increase the Ir−O covalency, while the high‐valence (pentavalent or higher) metal incorporation decreases the Ir−O covalency resulting in worse OER activity. Experimental and theoretical analyses indicate that enhanced Ir−O covalency activates lattice oxygen and triggers lattice oxygen‐mediated mechanism to enhance OER kinetics, which is verified by the finding of a linear relationship between the natural logarithm of intrinsic activity and Ir−O covalency described by charge transfer energy. By regulating the Ir−O covalency, the obtained Gd‐IrO_2–δ merely needs 260 mV of overpotential to reach 10 mA cm⁻² and shows impressive stability during a 200‐h test in 0.5 м H₂SO₄. This work provides an effective strategy for significantly enhancing the OER activity of the widely used IrO₂ electrocatalysts through the rational regulation of Ir−O covalency.