One possible scheme ofRugulopteryx okamuraebiomass valorization based on a green, rapid and efficient fractionation technique was proposed. Microwave-assisted pressurized hot water extraction was the technology selected as the initial stage for the solubilization of different seaweed components. Operation at 180 °C for 10 min with a 30 liquid-to-solid ratio solubilized more than 40% of the initial material. Both the alginate recovery yield (3.2%) and the phenolic content of the water-soluble extracts (2.3%) were slightly higher when distilled water was used as solvent. However, the carbohydrate content in the extract (60%) was similar for both solvents, but the sulfate content was higher for samples processed with salt water collected from the same coast as the seaweeds. The antiradical capacity of the extracts was related to the phenolic content in the extracts, but the cytotoxicity towards HeLa229 cancer cells was highest (EC50= 48 µg/mL) for the extract obtained with distilled water at the lowest temperature evaluated. Operation time showed a relevant enhancement of the extraction performance and bioactive properties of the soluble extracts. The further fractionation and study of this extract would be recommended to extend its potential applications. However, due to the low extraction yield, emphasis was given to the solid residue, which showed a heating value in the range 16,102–18,413 kJ/kg and could be useful for the preparation of biomaterials according to its rheological properties.