In this study, the biosorption of cobalt (II) ions on Sargassum sp., brown algae, was investigated in a batch system. The combined effects of operating parameters such as biomass treatment, initial pH, temperature, initial metal ion concentration, and biosorbent dosage on the biosorption of cobalt (II) ions on Sargassum sp. were analyzed using response surface methodology. The Mg(NO3)2 was used as a pretreatment agent for Sargassum sp. The optimum biosorption conditions were determined as Mg-treated biomass, initial pH 7.0, temperature 45 °C, biosorbent dosage 0.1 g, and initial cobalt (II) concentration 300 mg/l. At optimum biosorption conditions, the biosorption capacity of Mg-treated biomass for cobalt (II) ions was found to be 80.27 mg/g after 90-min biosorption. The Langmuir and Freundlich isotherm models were applied to the equilibrium data. The results are best fitted by the Freundlich model.