In this paper is reported an investigation aimed to the development of a relative humidity QCM (Quartz Crystal Microbalance) sensor based on PEDOT-PSS (poly(3,4-ethylendioxythiophene)-poly(styrenesulfonate)) polymer film. A home-made testing device based on microcontroller digital input ports, has been also developed to measure the QCM sensor frequency. Humidity tests for characterizing the developed sensor were carried out in the range 0-100% RH, at room temperature. Results obtained indicate that PEDOT-PSS has the required characteristics for measuring relative humidity with gravimetric-type devices. The developed QCM sensor is in fact very effective in a wide relative humidity range, showing good sensitivity, short response time and linearity in the entire humidity range tested. These good features were attributed to the peculiar morphology of the PEDOT-PSS film, showing a high porosity with the majority of pore having a diameter in the range between 1 and 10 μm.