Low-cost　and　ecofriendly　porous　biocarbons　were　fabricated　from　konjaku　flour　via　precarbonization　and　potassium　hydroxide　(KOH)　activation.　The　obtained　biocarbon　ACK-5　derived　from　a　precarbonized　carbon/potassium　hydroxide　(KOH)　mass　ratio　of　1:5　possessed　an　ultrahigh　specific　surface　area　of　1403　m2　g-1　and　hierarchical　porous　structures　with　the　existence　of　micro-　to　macropores.　When　ACK-5　was　employed　as　a　supercapacitor　electrode　in　6　M　KOH,　it　showed　a　high　specific　capacitance　of　216　F　g-1　and　excellent　cycling　stability　with　capacitance　retention　remaining　93.7%　after　5000　cycles.　Moreover,　the　ACK-5　sample　acquired　a　supramaximal　specific　capacitance　of　609　F　g-1,　and　the　high　energy　density　of　ACK-5//ACK-5　symmetrical　cells　reached　up　to　9.2　Wh　kg-1　when　p-phenylenediamine　serving　as　a　redox　electrolyte　was　added　into　KOH　electrolyte.　The　reported　simple　fabrication　strategy　would　leverage　a　green　biomass　precursor　for　the　preparation　of　supercapacitors.　©　Copyright　2018　American　Chemical　Society.