Magnesium silicate functionalized with tetrabutyl ammonium dihydrogen phosphate was developed as a new adsorbent for La(III) removal from aqueous solutions. The functionalized material was characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray analysis, and determination of the specific surface area (194 m²·g^–1). The influence of several experimental parameters (amount of adsorbent, contact time, temperature,and initial concentration of lanthanum ions) on the adsorption process was examined. The optimum dose of adsorbent was established as 4 g·L^–1. The optimum contact time of 15 min ensured a ∼95% La(III) removal efficiency at 298.15 K. The pseudo-second-order kinetic model was the best fit of the experimental data. The positive value of activation energy (10.7 kJ·mol^–1) suggested that the adsorption process was endothermic and the mechanism was chemisorption. Thermodynamic parameters (ΔG°, ΔH°, ΔS°) showed that the adsorption process was endothermic and spontaneous. Nonlinear regression analysis of the equilibrium data showed that the experimental data were well described by the Sips model. The maximum adsorption capacity was 9.13 mg·g^–1. The obtained results demonstrated that magnesium silicate functionalized with tetrabutyl ammonium dihydrogen phosphate is suitable for use as adsorbent for the removal of La(III) ions from aqueous solutions.