The marine biomaterial Posidonia oceanica (L.) fibres were used as a novel low-cost biological adsorbent for the removal of reactive textile dye (Cibacron Red) from aqueous solutions. Batch experiments were carried out for sorption kinetics and isotherms. Operating variables studied were contact time, fibres quantity, temperature, pH, and chemical pre-treatment. The equilibrium state was reached within 48 h. Biosorption capacity seems to be enhanced by, increasing the biosorbent mass. Rising the temperature has also a positive effect on dye removal rate. Maximum colour removal was observed at pH 5.5. Pre-treating fibres with H3PO4 and HNO3 solutions increased considerably the adsorption capacity. Kinetic and equilibrium data for raw fibres were well described by the pseudo-second order and Freundlich models, respectively. Besides, the thermodynamic study has showed that the dye-adsorption phenomenon onto P. oceanica biomass was favourable, endothermic and spontaneous.