Pressurized liquids, PLE, and enzyme-assisted extraction, EAE, have been tested to improve the extraction of phlorotannins from the seaweed Sargassum muticum. Enzymatic treatment with proteases and carbohydrases, alkaline hydrolysis and PLE with ethanol:water as extracting solvent have been studied in terms of extraction yield, total phenolic content and antioxidant activity (TEAC assay). Results demonstrated that the application of PLE alone provided the highest yields and relevant antioxidant activity. An experimental design was employed to further optimize the PLE extraction conditions; optimum parameters included the use of 160 °C and 95% ethanol. Under these conditions, values of 21.9%, 94.0 mg gallic acid equivalents g−1, 5.018 mg phloroglucinol equivalents g−1 and 1.275 mmol trolox equivalents g−1 were obtained for extraction yield, total phenols, total phlorotannins and TEAC, respectively. A preliminary chemical characterization by liquid chromatography coupled to mass spectrometry provided insight in terms of the mechanisms involved in the different processes.