A method for humidifying a reactant in a fuel cell system is provided having a fuel cell stack, which is fluidically connected to a humidifier, wherein the humidifier comprises a membrane, on whose surface channels are formed. At least one of the channels is associated with a storage element for temporary storing of liquid water, the method involving the following steps: extracting the liquid water from the fuel cell stack and feeding the liquid water to the humidifier, admitting at least part of the liquid water into the storage element and temporarily storing the part therein, at least partially emptying the storage element by evaporating of the liquid water and humidifying of the reactant being supplied to the fuel cell stack by means of the evaporated liquid water, wherein the liquid water is extracted from the fuel cell stack both at the anode side and at the cathode side. A fuel cell system for carrying out the method is also provided.