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.