A series of monolithic open-cellular hydrogel adsorbents based on carboxymethylcellulose (CMC) were prepared through high internal phase emulsions (HIPEs) and used to enrich the rare-earth metals La³⁺ and Ce³⁺. The changes of pore structure, and the effects of pH, contact time, initial concentration on the adsorption performance were systematically studied. The results show that the as-prepared monolithic hydrogel adsorbents possess good open-cellular framework structure and have fast adsorption kinetics and high adsorption capacity for La³⁺ and Ce³⁺. The involved adsorption system can reach equilibrium within 30 min and the maximal adsorption capacity is determined to be 384.62 mg/g for La³⁺ and 333.33 mg/g for Ce³⁺. Moreover, these porous hydrogel adsorbents show an excellent adsorptive reusability for La³⁺ and Ce³⁺ through five adsorption–desorption cycles. Such a pore hierarchy structure makes this monolithic open-cellular hydrogel adsorbent be an effective adsorbent for effective enrichment of La³⁺ and Ce³⁺ from aqueous solution.