Ruthenium dioxide (RuO₂) is a promising and cost-effective substitute for iridium oxide in the acidic oxygen evolution reaction (OER), but its application is hindered by the intrinsic trade-off between catalytic activity and stability due to the dissolution of high-valent metal sites. Herein, we design a heavy p‑block element Bi-modified RuO₂ electrocatalyst in which the lone‑pair electrons of Bi stabilize low‑valence Ru and introduce tensile strain, yielding weakened metal–oxygen covalency and overcoming the activity–stability limitation. In situ spectroscopies confirm that the Bi lone‑pair inhibits Ru overoxidation, ensuring outstanding structural stability of Bi_0.2Ru_0.8O₂, and simultaneously promote the formation of the pivotal *OOH intermediate, confirming a stable adsorbate evolution pathway instead of the traditional lattice oxygen mechanism. This highlights the critical role of bond covalency in stabilizing surface Ru and oxygen sites for enhanced acidic‑OER durability. As a result, the Bi_0.2Ru_0.8O₂ electrocatalyst delivers an ultralow overpotential of 269 mV at 100 mA cm^-2 and a small Tafel slope of 65.8 mV dec^-1. When implemented as the anode catalyst in a practical proton-exchange membrane water electrolyzer, Bi_0.2Ru_0.8O₂ enables a current density of 3.0 A cm^-2 at a cell voltage of only 1.837 V and sustains stable operation for 40 h at 200 mA cm^-2.