Generation 1 amine terminated polyamidoamine (PAMAM) dendrimer (G1-NH2)-Au nanoparticles (G1-NH2-AuNPs) was coated on a polyester (PET) substrate to form a novel flexible impedance-type humidity sensor. The formation of AuNPs was characterized by UV–vis spectroscopy. The microstructure of the G1-NH2-AuNPs film was analyzed by atomic force microscope (AFM). The effect of the AuNPs on the electrical and humidity sensing properties of the G1-NH2-AuNPs films on a PET substrate was investigated. The sensor had good flexibility. This flexible humidity sensor also exhibited good sensitivity and acceptable linearity (Y = −0.045X + 7.866; R2 = 0.9693) between logarithmic impedance (log Z) and RH in the range 30–90%RH, negligible hysteresis (within 2%RH), good response time (40 s) and recovery time (50 s), and long-term stability (39 days at least), measured at 1 V, 1 kHz and 25 °C. The flexible humidity sensor's linearity depended on the applied frequency. The temperature influence between 15 and 35 °C; found to be −0.55%RH/°C for 30–90%RH. The different complex impedance plots obtained at low and high relative humidity indicated that the ions dominate the conductance of the G1-NH2-AuNPs film.