This work describes the development of a core–shell magnetic nanoadsorbent (Fe₃O₄@TiO₂) designed for the selective recovery of rare earth elements (REEs) from electronic waste. The synthesis involved the co-precipitation of magnetite coated with an anatase-phase TiO₂ layer, subsequently functionalized with organophosphorus groups using glycolic acid and phosphoric acid. This surface modification, verified via FT-IR spectroscopy and zeta potential analysis, provided the material with a high density of active sites. Adsorption studies with lanthanum revealed that the process follows pseudo-second-order kinetics, reaching equilibrium in only 15 min with a theoretical model-calculated capacity of 19.4 ± 0.8 mg/g at pH 5. The material demonstrated high stability and reusability, maintaining 75% of its adsorption capacity after five cycles with a corresponding H₂SO₄ desorption efficiency of 58–60%. Finally, the nanoadsorbent was validated on real LCD screen leachates following an upstream pH 5.0 pre-neutralization and filtration stage designed to remove massive baseline concentrations of iron and copper. Although residual copper and chromium acted as the primary competitors within the remaining complex matrix, the material effectively partitioned REEs (Gd, Y, Ce, Pr, Nd, and Sm) present at ultra-low trace levels (μg/L), demonstrating its potential for urban mining and the circular economy.