Oxygen-rich pyromellitic acidic anions (PMA4-) have been intercalated into MgAl-layered double hydroxides to fabricate the MgAl-PMA-LDH (abbr. PMA-LDH) composite exhibiting excellent adsorption performance toward uranium (U(VI)). Benefiting from the large number of functional groups of -COO-, the PMA-LDH displays an extremely large maximum U adsorption capacity (qUm) of 352 mg g-1 and an ultra-fast sorption rate, reaching uptakes of ∼97% within 30 min and >99% in 1 h at the initial U concentration (CU0) of 113 ppm. Over a very wide pH range of 5-11, high U removals (>93%) are achieved at CU0 = 105 ppm. Moreover, in the presence of highly concentrated competitive ions, ultra-high selectivity of UO22+ is observed, giving a very large distribution coefficient (Kd) of ∼106 mL g-1. Moreover, the PMA-LDH exhibits effective capture of UO22+ in contaminated simulated seawater, showing high uptakes of >93% at CU0 ∼ 10 ppm and >98% at CU0 ∼ 100 ppm. The dispersion effect of LDH layers may contribute to the increase of U adsorption capacity, and the confinement effect of LDH is conducive to the improvement of sorption selectivity toward U. The exploration of the interaction mechanism of UO22+ with PMA4- confined within the LDH gallery offers an important basis for the fabrication of new kinds of organic/inorganic hybrid materials. The PMA-LDH is a highly effective adsorbent which can be applied to uranium extraction from seawater and uranium disposal in nuclear wastewater.