Nitrate (NO3−) pollution poses a critical environmental threat by contaminating water resources and disrupting the global nitrogen cycle. The electrochemical nitrate reduction reaction (NO3RR) in alkaline media offers a dual solution: mitigating nitrate contamination while enabling sustainable ammonia (NH3) production. However, the scarcity of free protons (H+) at high pH hampers efficient NO3−-to-NH3 conversion. Here, we report a sub-stoichiometric covalent organic framework PEPy-2CHO-TTA, synthesized by microwave-assisted [4 + 3 + 2] polycondensation strategy, which retains pendant unreacted aldehyde groups oriented toward the pore channels. This framework-intrinsic integration of polar aldehyde functionalities enhances water uptake and promotes the formation of a structured hydration network within the pores, enabling localized proton transfer that overcomes proton deficiency under alkaline conditions. As a result, PEPy-2CHO-TTA COF achieves a Faradaic efficiency (FE) exceeding 95% and an NH3 yield rate of 5.87 mg h−1 cm−2 which is among the highest reported for metal-free or metal-based porous electrocatalysts. Isotope labelling using K15NO3 confirms that the produced ammonia originates exclusively from nitrate reduction. DFT calculations reveal a multi-step eight-electron reduction pathway with the NO-to-NHO transformation as the potential-determining step. This work introduces a new design paradigm for COF electrocatalysts, where pendant aldehydes within the framework serve as molecular handles for water-mediated proton transport, enabling efficient nitrate reduction under alkaline conditions, without external acidification or metal catalysts.