Based on the unique fivefold equatorial coordination of UO₂ ²⁺, water-compatible pentadentate planar ligands, H₂saldian and its derivatives, were designed for the strong and selective capture of UO₂ ²⁺ in seawater. In the simulated seawater condition (0.5 M NaCl + 2.3 mM HCO₃ ^–/CO₃ ^2–, pH 8), saldian^2– shows the strongest complexation with UO₂ ²⁺ to form UO₂(saldian) (log β₁₁ = 28.05 ± 0.07), which is more than 10 order of magnitude greater than amidoxime-based or -inspired ligand systems most commonly employed for U capture from seawater. Good selectivity for UO₂ ²⁺ from other metal ions coexisting in seawater was also demonstrated.