Exploring　high-performance　and　low-priced　electrode　materials　for　supercapacitors　is　important　but　remains　challenging.　In　this　work,　a　unique　sandwich-like　nanocomposite　of　reduced　graphene　oxide　(rGO)-supported　N-doped　carbon　embedded　with　ultrasmall　CoNiSx　nanocrystallites　(rGO/CoNiSx/N-C　nanocomposite)　has　been　successfully　designed　and　synthesized　by　a　simple　one-step　carbonization/sulfurization　treatment　of　the　rGO/Co-Ni　precursor.　The　intriguing　structural/compositional/morphological　advantages　endow　the　as-synthesized　rGO/CoNiSx/N-C　nanocomposite　with　excellent　electrochemical　performance　as　an　advanced　electrode　material　for　supercapacitors.　Compared　with　the　other　two　rGO/CoNiOx　and　rGO/CoNiSx　nanocomposites,　the　rGO/CoNiSx/N-C　nanocomposite　exhibits　much　enhanced　performance,　including　a　high　specific　capacitance　(1028.2　F　g-1　at　1　A　g-1),　excellent　rate　capability　(89.3%　capacitance　retention　at　10　A　g-1)　and　good　cycling　stability　(93.6%　capacitance　retention　over　2000　cycles).　In　addition,　an　asymmetric　supercapacitor　(ASC)　device　based　on　the　rGO/CoNiSx/N-C　nanocomposite　as　the　cathode　and　activated　carbon　(AC)　as　the　anode　is　also　fabricated,　which　can　deliver　a　high　energy　density　of　32.9　W　h　kg-1　at　a　power　density　of　229.2　W　kg-1　with　desirable　cycling　stability.　These　electrochemical　results　evidently　indicate　the　great　potential　of　the　sandwich-like　rGO/CoNiSx/N-C　nanocomposite　for　applications　in　high-performance　supercapacitors.　©　2018　The　Royal　Society　of　Chemistry.