In this paper, the use of cold plasma-treated and formaldehyde-treated onion skins as a biosorbent has been investigated to remove methylene blue dye from aqueous solutions. The surface characteristics of the treated onion skins were investigated using Fourier Transform–infrared spectroscopy. The influence of process variables such as adsorbent dosage, initial dye concentration and pH were studied. Equilibrium isotherms were analysed by Langmuir, Freundlich, Temkin and Dubinin–Radushkevich isotherm models. The results indicated that the data for adsorption of methylene blue onto onion skins fitted well with the Langmuir isotherm model. The sorption capacities for cold plasma-treated and formaldehyde-treated onion skins by Langmuir isotherm were found to be 250 and 166.67 mg/g, respectively. The equilibrium time was found to be 150 min for 50 mg/l dye concentrations. The maximum removals for cold plasma-treated and formaldehyde-treated onion skins obtained were 90.94 and 95.54% at natural pH 10.0 for adsorbent doses of 0.15 g/200 ml, respectively. The rates of sorption were found to conform to pseudo-first-order kinetics. Results indicated that onion skins could be used as a biosorbent to remove methylene blue dye from contaminated waters.