Novel QCM NH₃ gas sensors made of reduced graphene oxide–SnO₂ (rGO–SnO₂) composite films were fabricated using a one-pot technique, which is a fast and easy fabrication route that is useful for practical applications. The surface morphology and composition of the rGO–SnO₂ composite films were analyzed using Fourier transform infrared spectroscopy, atomic force microscopy, scanning electron microscopy coupled with energy-dispersive X-ray elemental mapping and transmission electron microscopy. On the basis of the microstructural observations, SnO₂ connected to rGO sheets and naked rGO sheets were observed on the surface of the rGO–SnO₂ composite films. The effects of the amount of added rGO sheets on the enhancement of the NH₃ gas sensing properties of the rGO–SnO₂ composite films were studied. The analysis of the adsorption dynamics (association constant) confirmed the higher sensitivity of the QCM NH₃ gas sensor based on the rGO–SnO₂ composite film than that of the sensor with the pristine SnO₂ film.