Effective and selective removal of radioactive metal ions from aqueous solutions is of great importance due to their harmful effects on humans and other living species. However, it is a big challenge for researchers to develop effective adsorbents with high selectivity and a wide pH application range even if some progress has been achieved. Herein, we report an anionic potassium organic framework (UPC-K1) with protonated dimethylamine (Me2NH2+) residing inside the rhomboid channels. The unique 3D firm structure of UPC-K1 is constructed by the cross-linking of the 2D arrangement using weak K-O bonds (2.9270 Å) and strong hydrogen bonds (1.6498 Å), which endows it with excellent chemical stability in organic solvents, boiling water, and aqueous solution in the pH range 3-10. Based on the cation exchange, depending on pore size selectivity, UPC-K1 shows excellent adsorption performance towards UO22+ in aqueous solutions at 298 K with the following characteristics: (1) effective removal in the pH range 3-10; (2) high selectivity over other metal cations; (3) a high adsorption capacity of 551.4 mg g-1; (4) a rapid adsorption equilibrium within 3 hours under stirring; and (5) effective adsorption at low concentrations, with a residual concentration of 0.69 ppm even at an initial concentration of 10.3 ppm after stirring for 24 hours. These results indicate the great potential of UPC-K1 in the treatment of uranium-containing nuclear wastewater.