Nagamo (Sargassum horneri) processing residue was extracted using a hot water extraction method at various temperatures and solvent/material ratios and analyzed for total sugar (TS). The extraction characteristics of TS were characterized by Peleg’s model, and the optimum extraction conditions for the Nagamo processing residue were obtained at 85°C at a solvent/material ratio of 5. At experimental temperatures and solvent/material ratios, the TS content increased rapidly for the first 2 h and then reached the equilibrium concentration. Additionally, the antioxidant activity (SOD) of the extraction was 60.8% or higher. Thus, it was concluded that the Nagamo processing residue contains biologically active polysaccharides, which can be obtained by using a transportable 400 L-batch extraction process with hot water.