This study examined the sorption behavior of methylene 25 blue (MB) on extracted cellulose from Posidonia oceanica; an abundant Mediterranean biomass. Physicochemical characterization of extracted material was performed by Boehm titration, pHZPC determination, Scanning Electron Microscopy (SEM) and spectroscopic analysis (UV–vis, XDR, FTIR and XPS). The pHZPC occurred around pH 3.5. The empirical kinetic data fitted very well the pseudo-second-order kinetic. The isotherm data fitted rightly to the Langmuir isotherm model one with a maximum adsorption capacity of 0.955 mmol g−1. It seems that electrostatic interactions play an important role during the MB species adsorption process. The departure of water molecules when samples dry gives away more complex interaction mechanisms between MB species and extracted cellulose surface groups.