An ionic liquid (IL), 1-butyl-3-methylimidazolium bromide ([C(4)mim](+)[Br](-)) was entrapped within the cavities of the robust metal-organic framework (MOF) UiO-66 via a ship in a bottle technique. To this end, 1-methylimidazole and 1-bromobutane were introduced sequentially to diffuse through the pores of UiO-66 to form the IL species [C(4)mim](+)[Br](-) inside the MOF structure (C(4)mim@UiO-66). It was characterized by XRD, N-2 adsorption-desorption isotherms, elemental analysis, SEM, and FTIR spectroscopy. [C(4)mim](+)[Br](-) remained entrapped within the cavities of UiO-66 by its bulky size and interaction with UiO-66 framework as confirmed by FTIR, TGA, and density functional theory calculations. The material was subsequently applied for Gd3+ adsorption in water. A pseudo-second-order kinetic behavior was observed and a practical adsorption equilibrium was attained after 3 h of contact time at 25 degrees C. The maximum adsorption capacity of pristine UiO-66 for Gd3+ in water (17 mg/g) was greatly increased in C(4)mim@UiO-66 (85 mg/g) because of the strong interaction between Gd(3+ )ions and the N-sites of the imidazole rings of the entrapped [C(4)mim](+)[Br](-) molecules. The thermodynamic analysis indicated endothermic adsorption for Gd3+ ions on C(4)mim@UiO-66 with Delta H degrees of 24.4 kJ/mol. C(4)mim@UiO-66 showed a high selectivity of 82% for Gd3+ against other metal ions (Na+, Ca2+ , Mg2+ , Al-3(+), and Fe3+) present in the solution. Further, C(4)mim@UiO-66 could be used for five repeated runs without any significant drop in the adsorption capacity.