The cost-effective activated carbons derived from Enteromorpha prolifera (E. prolifera) were obtained via a single-step, quick microwave irradiation method. The activated carbon (AC-4) obtained at the radiation power of 800 W and the ZnCl2/E. prolifera mass ratio of 4 shows the maximum specific capacitance of 214F g(-1) at 0.1 A g(-1) and offers a good capacitance retention of ca. 91.2% after 1000 cycles at 0.5 A g(-1), suggesting its potential application in supercapacitors.