Cladophora glomerata (C. glomerata) biochar (raw biochar) with carbon-rich content and highly ordered micro/macro porous was prepared via slow pyrolysis technique and activated through a novel activation process with nitric acid. Then, both raw and treated biochar were characterized by XRD, FT-IR, FESEM-EDS, XPS, and BET analyses and used as a precursor to manufacture the working electrode. These electrodes demonstrate wonderful rate capability, high specific capacitance, and excellent cycle stability when first served as electrodes for supercapacitors. The treated biochar showed improved specific capacitance 376.7 F g(-1) at the current density of 1 A g(-1) with an energy density of 42.4 Whkg(-1), while the specific capacitance of raw biochar at the same current density was relatively high (197.8 F g(-1)) with an energy density of 22.25 Whkg(-1). The treated biochar electrode was also strongly stable showing only a 0.8% reduction in capacitance after 5000 successive potential cycles. Finally, the results of the study were compared to those obtained from carbon derived from agricultural wastes, terrestrial biomass, and organic wastes. The biochar derived marine biomass and treatment process developed here could provide a promising path for the low-cost, renewable and environmental friendly electrode materials.