For uranium extraction from seawater, development of new stable and reusable sorbents with high affinity and good selectivity for U(VI) is required. Herein, a new phosphate-functionalized polyethylene (denoted PO4/PE) was synthesized via a simple Ar-jet plasma treatment of PE in concentrated H3PO4 and was employed in U(VI) extraction from seawater. The prepared PO4/PE shows superior performance in the extraction of trace U(VI) from seawater. The adsorption process followed the second-order kinetics model and the Langmuir model. The maximum adsorption capacity of PO4/PE for U(VI) reaches 173.8 mg/g at pH 8.2 and 298 K. PO4/PE can be effectively regenerated by 0.1 mol/L Na2CO3 and reused well even after eight cycles. Experimental results offer a new approach for U(VI) uptake from seawater.