We fabricated a surface acoustic wave NH3 gas sensor based on a TiO2 sensitive film. The TiO2 film was deposited using a combined sol-gel and spin-coating technology. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) results indicate that the film was porous and had good crystallinity. Fourier Transform infrared spectroscopy (FTIR) analysis revealed that there was a large amount of hydroxyl groups on the film, which can capture H2O molecules from the ambient environment. The sensor showed a positive response to NH3 gas and the response increased significantly with increasing relative humidity. The positive response was found to be caused by the change in the elastic modulus of the sensitive film, which was induced by the condensation of the hydroxyl groups on the film catalyzed by NH3. The sensor also had a low detection limit of 1 ppm and excellent selectivity and stability to NH3 gas.