Highly concentrated aqueous electrolytes enable electrochemical performance beyond the limits imposed by water reactivity through solvation structure modulation. Molecular dynamics simulations reveal that in 7–8 m LiTFSI water-in-salt electrolytes, Li⁺-coordinated water stabilizes protonated pyrazine radical intermediates by suppressing aggregation and degradation, an effect absent in concentrated LiNO₃ or LiCl. In contrast, highly concentrated acidic LiNO₃ (>6 m) forms a “hydronium-in-salt” electrolyte, activating nitrate reduction on Pt while suppressing hydrogen evolution. A mixed solvation structure of Li⁺, NO₃⁻, and H₃O⁺ brings reactants into close proximity, facilitating proton-coupled electron transfer and demonstrating solvation control as a general strategy for regulating aqueous electrochemical reactions.