The development of an effective and reliable sensor with the capability to detect ammonia (NH₃) gas at room temperature exerts a significant influence on the sensor industry. The gas sensing performance is notably improved by the formation of a heterostructure between metal oxide with metal sulfides. In this study, pure ZnIn₂S₄ (ZIS), Cu₂O and heterostructures of ZIS with 5, 10 and 20 wt% of Cu₂O were successfully prepared using hydrothermal, co-precipitation and heat treatment methods, respectively. A thorough investigation has been carried out to examine the sensing capabilities of all the materials upon exposure to NH₃ with different concentrations (1, 5, 10, 15, 20, 25 and 50 ppm) at room temperature (RT). Impressively, the composite material 0.9ZnIn₂S₄/0.1Cu₂O (ZIS-10) has exhibited remarkable gas sensitivity compared to pristine ZIS and Cu₂O towards 25 ppm NH₃, low limit of detection (1 ppm) with fast response/recovery times (37/25 sec). The improved performance of the ZIS-10 composite sensor may be ascribed to the synergistic effect between ZIS and Cu₂O, which facilitates the electron transfer from ZIS to the Cu₂O at the interface. The plausible gas-sensing mechanism and the pathways responsible for enhanced sensing are also discussed in detail.