Pt-loaded In2O3 mesoporous nanofibers (NFs) have been prepared by electrospinning, followed by a facile reduction method. The sample obtained by adding tea saponin into the spinning solution, sample In-TS, displays a more than 4 times higher specific surface area, value of 86.9 m2 g−1, than that of conventionally prepared samples. The loading with small-sized Pt nanoparticles (NPs) determines excellent gas sensing performance towards NO2 down to ppb level (Rg/Ra = 2.8 to 10 ppb and 23.9–1 ppm) at room temperature (RT), by that significantly lowering the power consumption. The reason is that the open pores and channels on the mesoporous NFs generated by the foaming agent not only promote the adsorption and desorption of NO2 molecules on the sensing layers, but also improves the transduction function of the sensors by ensuring that there are no parts of the fiber that are not accessible to the gases. The introduction of Pt to the mesoporous NFs could accelerate the dissociation of absorbed oxygen and the active surface reactions in NO2 owing to the catalytic property and chemical sensitization of well-dispersed Pt NPs. Due to the unique gas sensing performance, sample In-TS-Pt has a prospect for applications in low-concentration NO2 detection at RT.