Ammonia as a toxic gas is becoming a significant concern for human health and ecological safety as its applications in daily life cannot be avoided. Ammonia vapor detection at very low doses is therefore required for its safe usage and handling. In this regard, materials at nanoscale offer increased surface area and easily controllable electrical/optical properties significant for developing sensor devices. In the present work, silicon nanowire (Si NW) coated with tin sulfide (SnS) to form heterojunction device has been synthesized and investigated for ammonia-sensing properties. One-dimensional vertically aligned Si NW films formed by metal-assisted chemical etching (MACE) have been used as the substrate to grow thin films of SnS via chemical bath deposition technique using triethylamine as the complexing agent. The as-prepared samples have been characterized using SEM, UV–Vis, and current versus voltage characteristics measured in presence/absence of ammonia vapors. The prepared sensor devices exhibited excellent stability and repeatability at room temperature conditions.