In the present study, pummelo peel pretreated with sodium hydroxide is used for adsorption of methylene blue (MB). In order to investigate the effects of pretreatment on the cationic dye adsorption, the kinetics and capacities of raw pummelo peel (RPP) and sodium hydroxide treated pummelo peel (TPP) to remove MB from aqueous solutions were compared. The effects of various experimental parameters (e.g., initial pH, dye concentration, contact time) were also studied. According to the maximum adsorption capacity (qm) obtained, adsorption capability of pummelo peel is significantly increased after sodium hydroxide treatment. Both adsorption isotherms of RPP and sodium hydroxide TPP fitted the Langmuir model well, but not the Freundlich model. The processes of uptake followed pseudo-first order rate kinetics. The results in this study indicated that sodium hydroxide TPP is a promising adsorbent for removing MB from aqueous solution.