Ionic covalent organic frameworks make up an emerging class of functional materials in which the included ionic interfaces induce strong and attractive interactions with ionic species of the opposite charge. In this work, the strong and selective binding forces between the confined diiminoguanidinium groups in the framework and tetrahedral oxoanions have led to unparalleled effectiveness in the removal of the toxic chromium(VI) pollutant from aqueous solutions. The new functional framework can take up from 90 to 200 mg/g of chromium(VI), depending on the solution pH, and is capable of decreasing the chromium(VI) concentration in water from 1 ppm to 10 ppb within minutes (an order of magnitude below the current U.S. Environmental Protection Agency maximum contaminant level of 100 ppb), demonstrating superior properties among known ion exchange materials and natural sorbents.