The potential use of a biosorbent, Cystoseira indica, obtained from the Persian Gulf was investigated for the removal of Th (IV) ions from aqueous solutions by considering equilibrium, kinetic and thermodynamic aspects. The FT-IR spectra of unloaded and Th-loaded biomass indicated various functionalities on the biomass surface including hydroxyl, amide and carboxyl groups, which are responsible for the binding of thorium ions. Th (IV) uptake by C. indica was pH dependent. An increase in biosorbent dosage up to 1 g/L caused an increase in the Th (IV) percentage removal. Biosorption process at all studied initial Th (IV) ion concentrations follows the pseudo-second order kinetic model. The biosorption data could be well described by Redlich-Peterson isotherm in comparison to Langmuir and Freundlich isotherms. The maximum sorption capacity of Th (IV) by Langmuir isotherm was estimated to be 169.49 mg/g at 45 ℃ with pH of 3. The thermodynamic parameters indicated the biosorption of Th on the biomass was a feasible, spontaneous and endothermic process. Th sorption capacity remained unaffected or slightly affected (<10% inhibition) in the presence of several interfering ions such as uranium (VI), nickel (II) and copper (II). The reusability of the biomass was also determined after five sorption-desorption cycles.