Seaweeds (also called macroalgae) are considered a potential biomass feedstock for biorefineries for production of energy and chemicals. In this study, a biorefinery strategy for the brown seaweed Saccharina latissima is described. Fresh S. latissima harvested at the Irish coast contained glucose and mannitol as most abundant fermentable sugars. The fresh biomass was chopped and pressed in order to obtain a liquid fraction (press juice), which contained 16 g/L of mannitol as main sugar component, and an insoluble fraction referred to a “press cake”. The mannitol in the press liquid has been extracted and purified to serve as a substrate for chemical conversions. The use of the press juice and hydrolysed press cake as substrates for production of acetone, butanol and ethanol by anaerobic fermentation has been evaluated. While the press juice was easily fermentable after addition of nutrients, the press cake was toxic for the microorganisms. When the press cake hydrolysate was diluted, fermentation was possible. The toxicity of the hydrolysate might be associated to the high salt concentrations determined in it. The use of the residue after enzymatic hydrolysis of the press cake as fertilizer has been evaluated.