Abstract It is well-known that in neutral and acidic aqueous electrolytes, MoS 2 monolayers can store charges by adsorption of cations on to the electrode-electrolyte interface as its analog of graphene. Restricted by its low conductivity and the charge storage mechanism, the electrochemical performance of MoS 2 monolayer supercapacitor electrode is not satisfactory. It is reported here that water bilayers absorbed on MoS 2 monolayers can be involved in charge storage. One proton of each absorbed water molecule can intercalate/de-intercalate the water bilayers during charging/discharging in the alkaline aqueous electrolyte. For two water molecules are present for every Mo atom, the water bilayers can endow MoS 2 monolayers an ultrahigh specific capacitance. In this paper, 1T phase MoS 2 nanosheets with three monolayers were synthesized by hydrothermal reaction. It presents a specific capacitance of 1120 F g −1 at a current density of 0.5 A g −1 in KOH. As it is assembled with active carbon into a hybrid supercapacitor, the device has an energy density of 31.64 Wh kg −1 at a power density of 425 W kg −1 , and gets a specific capacitance retention of 95.4% after 10,000 cycles at 2 A g −1 .