In the present study, the phosphorylated chitosan (CSP) and phosphate-decorated carboxymethyl cellulose (CMCP) were cross-linked to synthesize a water-stable CSP-CMCP composite. Fourier transform infrared spectroscopy (FTIR) analyses indicated the occurrence of a dehydration-condensation reaction between amino and carboxyl groups as well as the introduction of abundant phosphate sites on CSP-CMCP surfaces. The maximum adsorption capacity of CSP-CMCP toward U(VI) (i.e., 977.54 mg/g at pH = 5.0 and T = 293 K) was superior to a series of adsorbents reported in the previous studies. In addition, CSP-CMCP showed an extremely high affinity for the selective capture of U(VI) from a simulated wastewater with multiple competing metal ions. The integrated analyses of X-ray diffraction, FTIR, X-ray photoelectron spectroscopy, X-ray absorption near edge structure, and extended X-ray absorption fine structure spectroscopy suggested that the predominant U(VI) species formed inner-sphere surface complexes with the active phosphate groups, while a small proportion of U(VI) was reduced to a lower U(IV) state by the amino sites. These research findings highlighted the potential applicability of the CSP-CMCP composite for the remediation of uranium-bearing wastewater.