Extraction of radioactive uranium (U(VI)) from seawater has recently received extensive attention in the nuclear energy field. In this study, to acquire more void space of an MOF as active points for improving adsorption capacity, GO–COOH/UiO-66 composites were designed via coordination of the carboxyl groups of GO with zirconium ion of UiO-66; this was included as a part of the rapid and effective two-step method. Transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) were used to determine the effectiveness of the synthesis of GO–COOH/UiO-66 composites. The GO–COOH/UiO-66 composites were investigated for adsorption of U(VI) from an aqueous solution and artificial seawater. The results showed that the GO–COOH/UiO-66 composites had a high adsorption capacity at a suitable seawater pH with a high removal rate of U(VI) at the ppb and ppm levels. The adsorption process closely fitted the Langmuir isotherm model and pseudo-second-order rate equation. Based on the FTIR spectroscopy, the change in pH before and after the adsorption of U(VI), and X-ray photoelectron spectroscopy (XPS), a possible sorption mechanism of U(VI) onto GO–COOH/UiO-66 composites was revealed. In addition, the adsorbent showed good chemical stability under the operating conditions for the adsorption–desorption of U(VI) from an aqueous solution, which indicated a promising potential for applications in the extraction of U(VI) in seawater.