Due to its abundance, macroalgae Saccharina japonica, which is a third-generation biomass, was liquefied to produce biopolyol using a crude glycerol and polyethylene glycol 300 (PEG300) solvent mixture in the presence of sulfuric acid as a catalyst. The reaction parameters, including catalyst loading, reaction time, and reaction temperature, were optimized to obtain high biomass conversion by applying response surface methodology (RSM). The optimal conditions for Saccharina japonica solvothermal liquefaction were determined at 4.25% catalyst loading, a reaction time of 65 min, and a reaction temperature of 160 °C, with a corresponding biomass conversion of 82.63%. Finally, biopolyurethane foam with a closed cell structure and polyhedral cell shapes with a similar cell wall thickness was successfully produced, and its temperature at 5% and 10% weight loss were 268 °C (Td5) and 300 °C (Td10), respectively, comparable to neat polyurethane. We clearly demonstrated that marine biomass could be used for production of biopolyols and biopolyurethane.