<p>The increasing global reliance on alternative water sources underscores the critical need for enhanced desalination efficacy. Covalent organic frameworks (COFs), with their ordered porosity and tunable architectures, hold immense potential for next-generation desalination membranes. However, current COF membranes often fail in efficient seawater desalination due to pore sizes largely exceeding hydrated monovalent ion dimensions. Here we present a structurally stable, ultramicroporous COF membrane for low-pressure reverse osmosis (RO) desalination, engineered through a hydrogen-bond fortification strategy. Rational introduction of phenolic hydroxyl adjacent to aldehyde moieties yielded β-ketoenamine configurations enriched with hydrogen bonds, promoting AB-stacking and enhanced crystallinity in Tp-Bth COF membranes. The resultant COF membranes achieved 99.6% sodium chloride rejection with 1.7 L m⁻² h⁻¹ bar⁻¹ water permeability at 15 bar, demonstrating high-performance low-pressure RO desalination. Notably, these membranes exhibited high acid resistance, retaining their initial performance after 30 days in a solution at pH 3. This work demonstrates a hydrogen-bond-mediated strategy to precisely tailor COF pore architecture for high-performance desalination.</p>