The paper investigates the electrical properties of the junction between the graphene oxide (GO) and some metals (Cu and Au). A simple humidity sensor, based on the adsorption of water molecules on the GO/surface in a GO/Cu junction, is described. The GO was drop-casted as a thin film, 10 μm in thickness, on the metallic substrate. The junction open-circuit voltage and short-circuit current behaviours versus the humidity level can be explained on the base of the charge carrier drift. The room temperature response of the GO/Cu water vapour detector, in terms of relative humidity in air, is compared against a well-known commercial hygrometer. The proposed device does not require external polarization, and it is inexpensive and easy to use.