The brown alga Cystoseira barbatula Kützing which is widely distributed in the Mediterranean Sea was used to prepare an alternative low cost biosorbent to remove methylene blue (MB) from aqueous solutions. To determine the equilibrium adsorption capacity, the effects of contact time, temperature, pH and adsorbent dosage were studied. The contact time was studied at six hours but after 210 minutes the adsorption reached equilibrium in all concentrations (5-100 mg L-1). Pseudo-first and pseudo-second order kinetic models were applied to the adsorption data. Pseudo first order model was not fitted to data however pseudo second order model was well in line with the adsorption data. Langmuir, Freundlich and Dubinin-Radushkevich isotherm models were also investigated. According to Langmuir isotherm, the maximum adsorption capacity was estimated as 38.61 mg g-1 at 35°C. Dubinin-Radushkevich equation was used to analyze the adsorption behaviour of the algal adsorbent in aqueous MB solutions. Thermodynamic parameters ΔG°, ΔH° and ΔS° were also calculated by using Van't Hoff Equation. The positive enthalpy indicated that the adsorption was in endothermic nature. The adsorption enthalpy values were found as 72.464; 36.595; 33.974; 25.102 kJ mol -1 at different concentrations of methylene blue (5, 20, 50, 100 mg/L, respectively). High negative values of Gibbs free energy revealed that adsorption was in a spontaneous nature.