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Seawater is rich in uranium resources, but the marine environment is complicated and changeable, as well as the presence of a lot of competing ions. These are disturbing factors that cannot be ignored in seawater uranium extraction. The synthesis of adsorbents with good stability, high selectivity and large adsorption capacity has been the focus of attention. The composite adsorbent MXene-AO@LZU1 was prepared using electrostatic self-assembly of transition metal carbides and nitrides (MXenes) with structurally stable covalent organic frameworks (COF-LZU1). The results of characterizations showed that MXene-AO@LZU1 was rich in functional groups and had an effective improvement in specific surface area. MXene-AO@LZU1 exhibited an efficient performance for [UO 2 (CO 3) 3 ]4- with an adsorption saturation capacity of 88.8 mg·g−1. The adsorption amount in the simulated seawater also reached 6.8 mg·g−1. MXene-AO@LZU1 had a good selectivity for uranium and outstanding stability under various extreme conditions. In addition, XPS showed that the adsorption of uranium by MXene-AO@LZU1 relied mainly on the complexation of oxygen/nitrogen-containing functional groups with [UO 2 (CO 3) 3 ]4-. This study explored the great potential of MXenes and COFs in seawater uranium extraction, indicating that MXene-AO@LZU1 was an efficient and stable adsorbent with good development prospects. This finding was of great importance for the application of composite materials in actual environmental cleanup, and could be extended to efficiently remove other pollutants by rational ligands design. [Display omitted] • MXene-AO@LZU1 composite material was obtained by electrostatic self-assembly method. • The saturation adsorption capacity for [UO 2 (CO 3) 3 ]4- was 88.8 mg·g−1. • The adsorption amount in the simulated seawater also reached 6.8 mg·g−1. • Abundant nitrogen/oxygen functional groups promote uranium removal. • Main removal mechanism included surface complexation. [ABSTRACT FROM AUTHOR]