Novel low-humidity sensors were fabricated by the in situ photopolymerization of polypyrrole/Ag/TiO2 nanoparticles (PPy/Ag/TiO2 NPs) composite thin films on a quartz-crystal microbalance (QCM). The characterizations of the thin films were analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The sensitivity increased with the doping amount of TiO2 NPs. The PPy/Ag/50 wt% of TiO2 NPs composite thin films exhibited excellent sensitivity (0.0246 −ΔHz/Δppmv at 171.1 ppmv), linearity (R2 = 0.9576) and a short response time (12 s at 55.0 ppmv). The low-humidity sensing mechanism was elucidated in terms of surface texture and the nanostructural morphology of the composite materials. Additionally, based on the dynamic analysis of adsorption, the association constants of water vapor molecules with PPy/Ag and PPy/Ag/50 wt% TiO2 NPs composite thin films were estimated to be 81.6 and 227.9 M−1, respectively, explaining the effect of adding 50 wt% TiO2 NPs to PPy/Ag; the sensitivity to low humidity increased as the association constant increased.