Novel flexible humidity sensors were fabricated layer-by-layer (LBL) by covalently bonding graphene oxide (GO) to a gold electrode on a plastic substrate using a peptide chemical protocol, and then partially reducing in-situ the GO film. The effects of the duration of reduction of the GO film on the electrical and humidity-sensing properties of the reduced GO film were investigated. The anchored partially reduced GO film was characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR). This flexible impedance-type humidity sensor exhibited a strong water resistance, a wide range of working humidities, a high sensitivity, an acceptable linearity, a small hysteresis, high flexibility, a short response/recovery time, a weak dependence on temperature and high long-term stability. The flexible humidity sensor's linearity depended on the applied frequency. The complex impedance plots that were obtained at low and high relative humidities revealed that the ions (H3O+) dominate the conductance of the anchored partially reduced GO film.