Novel flexible impedance-type humidity sensors that were based on gold nanoparticles (AuNPs) and graphene oxide (GO) were fabricated by a combination of self-assembly and the sol–gel technique. A hydrolyzed 3-mercaptopropyltrimethoxysilane (MPTMOS) sol–gel solution that contained GO was firstly dropped on to the surface of a pair of comb-like Au electrodes on a polyethylene terephthalate (PET) substrate, and then AuNPs were assembled onto the thiol groups of the sol–gel network. The formed AuNPs were characterized by UV–vis spectroscopy. The microstructure of the AuNPs/GO/MPTMOS sol–gel film was analyzed by atomic force microscope (AFM) and scanning electron microscopy (SEM). The effects of the AuNPs and the amount of GO added on the flexibility, electrical and humidity sensing properties of the AuNPs/GO/MPTMOS sol–gel films on a PET substrate were investigated. The sensor that was made from the AuNPs/GO/MPTMOS sol–gel film with 9.0 wt% added GO exhibited the greatest flexibility, sensitivity, linearity and long-term stability. The effects of applied frequency, ambient temperature, response and recovery times on the impedance of the flexible humidity sensor were also investigated. The sensing mechanism of the AuNPs/GO/MPTMOS sol–gel film was explained with reference to impedance plots.