The polyaniline (PANI) and Au-loaded mesoporous In₂O₃nanospheres@PANI core-shell nanohybrid (PAInxAy) sensing materials with different amounts of mesoporous In₂O₃and Au-loaded mesoporous In₂O₃(2–50 mol%) nanospheres were synthesized through combination methods of facile hydrothermal and in-situ chemical oxidative polymerization. The loading amount of Au was varied from 0.5 at% to 2.0 at%. The sensing materials were performed on flexible polyethylene terephthalate (PET) substrate to fabricate the sensing device and investigate the gas sensing performances at room temperature. Various characterization techniques including FESEM, TEM, FTIR, XRD and XPS were performed to identify the morphology, structure and element information of the synthesized materials. The sensing performances demonstrated that the sensor utilizing 1 at% Au-loaded 20 mol% mesoporous In₂O₃nanospheres@PANI core-shell nanohybrid (PAIn20A1) showed the highest response value (∼46) to 100 ppm NH₃at room temperature, which were about 14 times and 4 times higher than those of pure PANI and PAIn10. The sensor also exhibited excellent selectivity, good reproducibility and perfect response-concentration linearity to NH₃at room temperature. Besides, the effect of relative humidity on the sensing performance and the sensing mechanism for the sensitivity enhancement were discussed.