• A cascade biorefinery process for Sargassum muticum (Sm) was proposed.
• Dried sap (2% of raw material) was enriched in K and stimulated stem and root growth.
• Similar alginate, phenolics and fucoidan yields and properties to those from unpressed Sm.
• Methane production from autohydrolysis solids increased by 55-76% compared to the use of Sm.
• The autohydrolysis at a maximum of 150 °C was optimal for different products.
Marine macroalgae represent an excellent material to be used as biogas producer, adsorbent, biostimulant and fertilizer for soils, or feedstock. The success in the exploitation of seaweeds depends on their characteristics, and the approach used to separate their specific active components. In the context of circular economy, invasive species are a good candidate for exploitation, and biorefinery a key valorization technique. Here we investigate a novel biorefinery scheme for a fuller valorization of the alien species Sargassum muticum. An initial pressing stage allowed the production of a Sap fraction, which showed potential as a plant biostimulant, increasing both root development and shoot/root ratio, especially when used at a dose of 0.1 g/L lyophilized Sap. The solids after pressing were processed by non isothermal autohydrolysis, using pressurized hot water under subcritical conditions (120-210 °C), previously optimized to solubilize the fucoidan and phlorotannin fractions. The residual solids remaining after pressing and autohydrolysis stages were evaluated for the production of biogas. The obtained value (150 mL CH4/g residual solids at 150 °C) is significantly higher than that found for the raw seaweed. The optimal autohydrolysis temperature (150 °C) is compatible with the production of the fucoidan fraction, although the phenolic content is favoured at stronger operation conditions. We also discuss the possibility of preparing adsorbents for pollutant removal and mineral amendments from the autohydrolysis waste solids.