The competition between uranium ions and other metal ions is one of the great challenges for recovery of uranium(VI) from seawater. In particular, vanadium ions that bind strongly to sorbents are hardly stripped off, largely decreasing the sorption capability of the sorbents for uranium. In this study, polyoxime was, for the first time, designed and conjugated onto magnetic nanoparticles for selective sorption of uranium(VI) against vanadium. Magnetic iron oxide nanoparticles were first grafted with polyglycerol on the surface, and then conjugated with oxime groups via esterification and oximation to give polyoxime-functionalized magnetic nanoparticles (POMNs). POMNs showed superior aqueous dispersibility and could be readily recovered by magnetic separation. The uranium(VI) sorption of POMNs achieved equilibrium in 5 min with a capacity of 141.4 mg g⁻¹ in a single-element solution of uranium at pH 8.0 and 298.15 K. The sorption in simulated seawater demonstrated that the POMNs possessed excellent selectivity for uranium(VI) against vanadium(V) and other competing metal ions. No significant change in sorption efficiency and selectivity for uranium was observed following four sorption–desorption cycles using Na₂CO₃ solution as an eluent. This work may unfold a new approach for selective sorption of uranium in seawater.