Ethanol production from lignocellulosic materials has been extensively studied in order to substitute fossil fuels. The aim of the study was to investigate the potential of Pocidonia oceanica residues as a potential source of fermentable sugars for bioethanol production in an integrated process. By consecutive supercritical CO2 and supercritical water (SCW) hydrolysis with a solid:liquid ratio of 0.08 (w/v) at 400 °C, cellulose and lignin were retained in solid phase whereas a reducing sugar value of 14.1 g/L was quantified in the liquid phase. Supercritical CO2 extraction acted as a pretreatment method in order to loosen the lignin structure and SCW hydrolysis was responsible for both releasing some of the hemicellulose in the matrix. The results are very promising in terms of deployment of the utilization processes to industrial scale applications, thereby proposing an alternative solution to the landfill of P. oceanica residues.