In this paper, a nickel oxide (NiO)/molybdenum disulfide (MoS2) heterostructure was successfully synthesized by an economical hydrothermal method, and was spin-coated on a substrate with interdigitated electrode as a sensing material to construct ammonia (NH3) sensor. The surface morphology and microstructure of the NiO/MoS2 nanocomposite were inspected by various characterization means. The gas-sensing investigations demonstrated that the proposed NiO/MoS2 film sensor has outstanding sensing abilities toward ammonia gas in terms of high response, good reversibility, acceptable selectivity and swift response/recovery properties. The unique morphology and the synergistic effect of the NiO/MoS2 heterojunction contributed a lot to its enhanced gas sensing properties. The NiO/MoS2 heterostructure film sensor has promising applications for NH3 detection at room temperature.