Huge reserves of uranium (U) in seawater have been of interest to scientists and energy companies since the 1950s. However, extracting trace concentrations (3.3 ppb) of U from seawater is economically unfeasible without new, high-performance adsorbents. Here, a mat-like nanofibrous composite adsorbent containing binary coordination groups (amidoxime (AO) and carboxyl (AC(-))) in a highly porous network of nanofibers is constructed via a parallel-blend electrospinning method. Its U uptake in artificial seawater is more than double those of adsorbents containing AO or AC(-) groups alone. Density functional theory (DFT) calculations reveal that this synergistic effect is because the AC(-) group promotes both the U 5f/6d orbital contribution to U-AO bonding and the dissociation of uranyl tricarbonate ions in seawater. In a continuous flow-through experiment with simulated seawater, the nanofibrous adsorbent achieves an adsorption capacity up to 2.86 mg U g(ads)(-1) in 30 d but without saturation, indicating a high efficiency for U extraction.