Resistive-based gas sensors have been considered as the most favorable gas sensors for detection of toxic gases and volatile organic compounds (VOCs) because of their simple structure, low cost, high sensitivity, ease of use, and high stability. Unfortunately, wide application of resistive-based gas sensors is limited by their low selectivity. In this article, we present the fabrication of ultrahigh selective NH₃ gas sensor based on tin–titanium dioxide/reduced graphene/carbon nanotube (Sn–TiO₂@rGO/CNT) nanocomposites. The Sn–TiO₂@rGO/CNT nanocomposites with different molar ratios of Sn/Ti (1:10, 3:10, and 5:10) were synthesized via the solvothermal method. Characterizations by scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy confirmed the decoration of Sn–TiO₂ nanoparticles on rGO/CNT nanocomposite surfaces. The Sn–TiO₂@rGO/CNT nanocomposite gas sensor exhibited high response and ultrahigh selectivity to NH₃ against toluene, dimethylformamide, acetone, ethanol, methanol, isopropanol, formaldehyde, hydrogen, carbon dioxide, acetylene, and VOCs in paint thinners at room temperature. The Sn–TiO₂@rGO/CNT nanocomposite gas sensor with molar ratio of Sn/Ti = 1:10 showed the highest response to NH₃ over other molar ratios of Sn/Ti as well as pure rGO/CNT and Sn–TiO₂ gas sensors. The ammonia-sensing mechanisms of the Sn–TiO₂@rGO/CNT gas sensor were proposed based on the formation of p–n heterojunctions of p-type rGO/CNT and n-type Sn–TiO₂ nanoparticles via a low-temperature oxidizing reaction process.