This article described relative humidity (RH) sensing using a ZnO-based film bulk acoustic-wave resonator (FBAR). The resonant frequency of the FBAR decreased in a two-stage manner as the RH increased. For low RH, a frequency downshift of 2.2 kHz per 1% RH change was observed. This effect was attributed to water molecules replacing the adsorbed oxygen on the ZnO surface, thus increasing the density of the film. For high RH, a frequency downshift of 8.5 kHz per 1% RH change was obtained, which was due to the mass loading effect of the water layers formed on the ZnO surface.