<p>Electrocatalytic oxygen reduction reaction (ORR) for H₂O₂ production represents a sustainable alternative route to the energy-intensive anthraquinone process. Nevertheless, under industrially-relevant acidic conditions, excessive protons at the reaction interface exacerbate low H₂O₂ selectivity and severe H₂O₂ reduction. Herein, we propose a universal alkali metal cation (AMC: Li⁺, Na⁺, K⁺, or Cs⁺) dosing strategy to markedly boost the acidic H₂O₂ electrosynthesis. Upon Cs⁺ addition, 2e⁻ ORR selectivity increases from 20% to 80%, concurrently suppressing an H₂O₂ reduction current by 50% and achieving an H₂O₂ production rate of 9.2 mol g⁻¹ h⁻¹ at 500 mA cm⁻². Microelectrode hydrogen evolution measurements witness impeded proton diffusion in AMC-dosed acidic electrolytes, directly restricting proton supply to catalytic active sites. In situ spectroscopic analysis combined with molecular dynamics simulation demonstrate AMCs help reconfigure interfacial water networks via cation hydration shells, thereby disrupting proton-hopping pathways. The efficacy trend (Li⁺ &lt;Na⁺ &lt;K⁺ &lt;Cs⁺) originates from distinct cation-specific interfacial water restructure, delivering mechanistic insights into cation-promoted selective H₂O₂ electrosynthesis in acidic media.</p>