The rapid and sensitive detection of trace ammonia (NH3) at room temperature is crucial for environmental monitoring, occupational safety, and noninvasive disease diagnosis. However, pristine two-dimensional tungsten disulfide (WS2) suffers from oxidative instability in air and sluggish room-temperature gas adsorption/desorption kinetics, limiting its sensing performance. Herein, a supercritical CO2 (SC CO2)-assisted heterointerface engineering strategy was developed to construct interface-purified WS2/reduced graphene oxide (rGO) heterostructures via a reductant-assisted one-step process. The results show that SC CO2 treatment at 180 °C, combined with the reductant, simultaneously promotes WS2 exfoliation, in situ reduction of GO, and interfacial purification, leading to an “ultrathin nanosheet–clean interface–strong electronic coupling” structure. TEM results confirm that WS2 nanosheets are uniformly anchored on rGO, while X-ray photoelectron spectroscopy (XPS) reveals markedly reduced W6+ and sulfate species. Benefiting from the purified heterointerface and enhanced interfacial charge transfer, the SC-WS2/rGO sensor exhibits fast response/recovery times of 11.4/11.7 s toward 6 ppm NH3 at room temperature, stable and reversible responses over the range of 6–900 ppm, a low theoretical detection limit of 13.11 ppb, negligible degradation after 30 days, and a selectivity up to 25 times higher than that toward interfering gases. Mechanistic analysis indicates that the clean heterointerface lowers the carrier transport barrier and exposes active sites, while electron delocalization provides efficient charge transport pathways. An effective interface engineering strategy for high-performance room-temperature NH3 sensors based on two-dimensional transition metal sulfides is demonstrated in this work.