Hierarchical porous carbons (HPCs) with high specific surface area are synthesized by a simple self-template strategy from the alage Enteromorpha prolifera (E. prolifera). The surface of the dried E. prolifera biomass contains carboxylic/hydroxyl groups and mineral salts, which can cooperate together to form metal-organic framework complexes. These salts and complexes can serve as self-templates to produce hierarchical porous structures during the activation process. The activated carbon is used to make an HPC electrode and the electrochemical properties of the supercapacitor fabricated from this HPC electrode are characterized by cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy in 6 mol L−1 KOH solution. The specific capacitance is 210 F g−1 at a current density of 3 A g−1. The good capacitive behavior is attributed to the high BET-surface area of 3332 m2 g−1, large pore volume of 2.46 cm3 g−1 and hierarchical porous structure (an abundance of interconnected mesopores, macropores and micropores). These results demonstrate that E. prolifera is a promising precursor to prepare high performance and low cost electrode materials for electrical double layer capacitors (EDLCs).