Abstract: Enteromorpha prolifera (EP) was carbonized, treated by HCl pickling to remove Ca²⁺ ions to form an "egg-box" structure, and activated by KOH to obtain a porous carbon (PC). The porous texture and electrochemical performance of the PC were compared with one produced without the HCl pickling stage. Results indicate that the HCl treatment leads to the formation of a porous structure with a high specific surface area (S_BET), up to 3 283 m² g⁻¹, with more than 66% of the surface area contributed by mesopores, while the carbon prepared without HCl treatment is microporous. The PC with the HCl treatment had an excellent electrochemical performance when used as the electrode material of a supercapacitor even at high current densities. Its gravimetric capacitance reached 361 F g⁻¹ at a current density of 0.1 A g⁻¹, and the capacitance remained at 323 F g⁻¹ at a current density of 10 A g⁻¹, both of which are higher than obtained using the PC without HCl treatment.