The biosorption of Ce(III) from aqueous solution by citric acid-modified Pinus brutia leaf powder was studied in a batch system as a function of initial pH, temperature, initial concentration of adsorbate, and contact time. Central composite design method was used in the experiments. Thermodynamic parameters such as standard enthalpy (Delta H (0)), entropy (Delta S (0)), and free energy (Delta G (0)) were calculated, and the results indicated that biosorption was exothermic. The biosorption of Ce(III) on modified Pinus brutia leaf powder was investigated by Freundlich, Langmuir, and Dubinin-Radushkevich (D-R) isotherms. The results show that Ce(III) adsorption can be explained by Langmuir isotherm model, and monolayer capacity was found as 62.1 mg/g. The results suggested that the modification process enhances the biosorption capacity of the adsorbent, and modified Pinus brutia leaf powder may find promising applications for the recovery of Ce(III) from aqueous effluents.