Self-nitrogen-doped　biocarbons　are　obtained　from　the　watermelon　rind　via　an　easy　one-step　pyrolysis-activation　process.　Porous　biocarbon　AC　W　-2,　obtained　with　a　watermelon/potassium　hydroxide　(KOH)　mass　ratio　of　1:2　at　700　°C,　exhibits　a　large　specific　surface　area　of　1303.3　m　2　g　−1　and　is　decorated　with　abundant　nitrogen/oxygen-containing　functional　groups.　When　AC　W　-2　is　used　as　a　supercapacitor　electrode,　a　high　specific　capacitance　of　260　F　g　−1　is　achieved　at　a　current　density　of　1　A　g　−1　.　AC　W　-2　also　exhibits　an　ultrahigh　cycling　stability,　and　its　retention　rate　is　maintained　at　93.8%　even　after　5000　cycles　at　a　scan　rate　of　100　mV　s　−1　.　Furthermore,　AC　W　-2　acquires　a　supramaximal　specific　capacitance　(852　F　g　−1　)　and　a　high　energy　density　(12.9　Wh　kg　−1　)　when　p-phenylenediamine　redox　additive　is　added　into　the　KOH　electrolyte.　Therefore,　the　watermelon　rind　can　be　used　as　a　promising　biomass　precursor,　and　the　as-prepared　porous　biocarbon　is　expected　to　be　an　outstanding　comprehensive　electrode　material　for　electrochemical　supercapacitors.　©　2019　WILEY-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim