The development of non-iridium-based oxygen evolution reaction (OER) catalysts is crucial for proton exchange membrane water electrolysis (PEMWE), but hydrogen production remains a great challenge because of sluggish OER kinetics and severe catalyst dissolution. Here, we present a 4f-induced covalent polarity modulation strategy for the construction of 4f-orbital-modified RuO2 (4f-RuO2) nanocatalysts with tunable Ru–O polarity. We find that the OER activity of 4f-RuO2 shows a volcano shape as a function of the polarity of Ru–O bond. Consequently, the best 4f-Nd-RuO2 catalyst possesses an ultra-low overpotential of 214 mV at 10 mA cm-2 and robust electrochemical stability in 0.1 M HClO4. Theoretical calculations coupled with in situ synchrotron infrared and X-ray absorption spectroscopy analyses reveal that the modulation of Ru–O polarity in RuO2 by the valence f−p−d gradient orbital coupling can modify the adsorption energy of the reaction intermediates and suppress the participation of lattice oxygen to avoid over-oxidation of Ru, which can thus serve as an effective descriptor for fine tuning the activity and durability of acidic OER nanocatalysts.