A hierarchically porous carbon material (HPCM) with plentiful pores from 0.5 to 700 nm and oxygen-enriched surfaces has been prepared starting from sodium alginate by using a sustainable and green process in which neither porous templates nor additional activation agents are utilized. While the macropores originate from the dissolution of in situ formed Na2CO3 particles during the carbonization of sodium alginate, the micropores and mesopores derive from the chemical activation process of Na ions in sodium alginate and the interspaces between the packed carbon nanoparticles, respectively. Raman spectrum and X-ray photoelectron spectroscopy reveal its nature of partial graphitization and oxygen-enriched functionalities. Electrochemical tests for electrochemical capacitors show that the present HPCM could deliver both higher energy and higher power densities than commercial activation carbon. The high energy density can be ascribed to the oxygen-enriched surfaces as well as the plentiful micropores of HPCM. While the former could provide large pseudocapacitance, the latter would strengthen the electric double layer capacitance. On the other hand, the high power density could be attributed to the excellent meso/macroporosity of HPCM.