Abstract: Traditional resistive semiconductor gas sensors suffer from high operating temperatures and poor selectivity. Thus, to address these issues, a highly selective nitrogen dioxide (NO₂) sensor based on lead sulfide (PbS) quantum dots (QDs)–lead molybdate (PbMoO₄)–molybdenum disulfide (MoS₂) ternary nanocomposites operating at room temperature was fabricated herein. The ternary nanocomposites were synthesized using an in situ method, yielding PbS QDs with an average size of ~10 nm and PbMoO₄ nanoparticles in the 10- to 20-nm range, uniformly distributed on ultrathin MoS₂ nanosheets with an average thickness of ~7 nm. The optimized sensor demonstrated a significant improvement in response to 1 ppm NO₂ at 25°C, achieving a response of 44.5%, which was approximately five times higher than that of the pure MoS₂-based sensor (8.5%). The sensor also achieved relatively short response/recovery times and full recovery properties. Notably, the optimal sensor displayed extraordinary selectivity toward NO₂, showing negligible responses to different interfering gases. Density functional theory (DFT) calculations were conducted to elucidate the underlying sensing mechanism, which was attributed to the enhanced specific surface area, the receptor function of both PbS QDs and PbMoO₄ nanoparticles, and the transducer function of MoS₂ nanosheets.