A method for starting a mobile fuel cell system, in particular for a vehicle, includes conveying air to be supplied to the system during starting and operation of the system by a motor-powered compressor, in order to ensure a necessary operating pressure and, if required, additional humidifying of the air. A motor drive for the compressor has an electric motor which is supplied from a mobile starter battery during a starting phase. An output voltage of the starter battery is different from an operating voltage at an output of a fuel cell stack of the fuel cell system. A control device is provided between an output of the starter battery or of the fuel cell stack and a motor input. The control device interrupts the starting voltage supplied from the output of the starter battery and switches over to the output voltage of the fuel cell stack when the output operating voltage of the fuel cell stack reaches an adequate level. A fuel cell system is also provided.