Bimetal sulfides as anode electrode materials have attracted extensive attention owing to their superior electrochemical activity compared to their mono-metal sulfide counterparts. Herein, ZnCoS nanomaterial was synthesized by chemical precipitation and ion-exchange process. The obtained ZnCoS can be considered as the product of partial substitution of Zn²⁺ by Co²⁺ and/or Co³⁺ ions in the ZnS lattice. Benefiting from the synergistic effects, the ZnCoS was evaluated as electrode material for supercapacitors. By varying the preparation conditions, we found that the ZnCoS material synthesized using an initial mole ratio of Co/Zn = 2 at 50 °C gave the best performance with a maximum specific capacitance of 1134.7 F g^-1 at 1 A g^-1, which is about 7.7 times that of bare ZnS electrode material. Furthermore, this electrode material exhibits good rate capability (81% retention from 1 to 20 A g^-1) and excellent cycling stability with no obvious specific capacitance decrease at 20 A g^-1 after 6000 charging-discharging cycles. A fabricated flexible asymmetric supercapacitor, consisting of ZnCoS and porous reduced graphene oxide, displays a maximum specific capacitance of about 90 F g^-1 at 10 mV s^-1 with an energy density of 17.7 W h kg^-1 at a power density of 435 W kg^-1.