CTS-g-(AA-co-SS)/ISC hybrid hydrogel adsorbent with crosslinked network structure and superior adsorption performance for rare-earth metal ions was successfully synthesized in aqueous solution by a simple one-step free-radical grafting polymerization reaction among acrylic acid (AA), sodium p-styrenesulfonate (SS) and chitosan (CTS) using illite/smectite clay (ISC) as the inorganic additive. The structure of the as-prepared CTS-g-(AA-co-SS)/ISC hydrogel adsorbent was characterized, and the reaction parameters such as AA/SS molar ratio and ISC content were optimized, and the effects of pH values, initial concentration and contact time on the adsorption performance for Ce(III) and Gd(III) were systematically evaluated. It was found that the maximum adsorption capacities of the hydrogel adsorbent toward Ce(III) and Gd(III) reached 174.05 and 223.79 mg/g, respectively, and the adsorption quickly achieved equilibrium within 15–20 min. The adsorbed Ce(III) and Gd(III) could be easily desorbed for recovery, and the used adsorbent was able to be regenerated for reuse. After five adsorption-desorption cycles, the regenerated adsorbent could still retain the adsorption capacities that were close to the initial value. The adsorption process was well described by pseudo-second-order kinetic mode and the Langmuir isotherm model, and the chemical complexation between ions and –COO^– was mainly responsible for the high adsorption capacity. As a whole, the hybrid hydrogel adsorbent was potential to be used for the adsorption and recovery of Ce(III) and Gd(III) from water.