Abstract: Recovery of rare earth elements (REEs) from secondary resources is an important route to alleviating constraints on primary ore supply, strengthening critical resource security, and reducing life-cycle environmental impacts. However, secondary resources usually contain low concentrations of REEs, various coexisting metal ions, and complex mineral matrices. In addition, changes in leachate acidity, ligand composition, and redox conditions can alter REE speciation during recovery. These factors hinder selective separation by intensifying ion competition, surface fouling, and mass-transfer resistance, and by making it difficult to translate high laboratory adsorption capacities into stable process performance. Adsorption provides a flexible strategy for enriching and separating REEs from complex, low-concentration systems through tailored surface functional sites and interfacial interactions. This review focuses on typical secondary resources, including phosphogypsum, red mud, and electronic waste, and examines their compositions, REE occurrence, leaching behavior, and separation challenges arising from coexisting metals and changing REE speciation. Against this background, the review summarizes the design principles, recognition mechanisms, and separation performance of polymer-, carbon-, silica-, porous-framework-, and bio-based adsorbents. The review highlights key mechanisms such as coordination, electrostatic attraction, ion exchange, chelation, size matching, and confinement effects. In addition to adsorbent design, this review discusses process-related issues, including leachate regulation, adsorbent shaping and mechanical stability, fixed-bed and membrane adsorption systems, mass-transfer enhancement, elution, regeneration, and impurity control. To bridge the gap between batch-scale material evaluation and engineering application, we propose a minimum information set for dynamic REE adsorption studies, covering feed composition, REE speciation, competing ions, pH and ionic strength, adsorbent form and particle size, breakthrough criteria, flow rate, bed geometry, utilization efficiency, eluent consumption, regeneration stability, and mass-balance closure. This minimum information set provides a basis for comparing adsorption systems and for linking material performance with process design. Future studies should move beyond static adsorption capacity and prioritize actual complex leachates, in situ or operando tracking of REE speciation, coupled adsorption–desorption behavior, long-term cycling stability, and techno-environmental assessment. Integrating these efforts with material design, selective interfacial recognition, process intensification, and standardized evaluation will be crucial for advancing efficient, green, and sustainable recovery of REEs from secondary resources.