Hierarchical In2O3@WO3 nanocomposites, consisting of discrete In2O3 nanoparticles (NPs) on single-crystal WO3 nanoplates, were synthesized via a novel microwave-assisted growth of In2O3 NPs on the surfaces of WO3 nanoplates that were derived through an intercalation and topochemical-conversion route. The techniques of XRD, SEM, TEM and XPS were used to characterize the samples obtained. The gas-sensing properties of In2O3@WO3 nanocomposites, together with WO3 nanoplates and In2O3 nanoparticles, were comparatively investigated using inorganic gases and organic vapors as the target substances, with an emphasis on H2S-sensing performance under low concentrations (0.5–10 ppm) at 100–250 °C. The results show that the In2O3 NPs with a size range of 12–20 nm are uniformly anchored on the surfaces of the WO3 nanoplates. The amounts of the In2O3 NPs can be controlled by changing the In3+ concentrations in their growth precursors. The In2O3@WO3 (In/W = 0.8) sample has highest H2S-sensing performance operating at 150 °C; its response to 10 ppm H2S is as high as 143, 4 times higher than that of WO3 nanoplates and 13 times that of In2O3 nanocrystals. However, the responses of the In2O3@WO3 sensors are less than 13 upon exposure to 100 ppm of CO, SO2, H2, CH4 and organic vapors, operating at 100–150 °C. The improvement in response and selectivity of the In2O3@WO3 sensors upon exposure to H2S molecules can be attributed to the synergistic effect of In2O3 NPs and WO3 nanoplates, hierarchical microstructures and multifunctional interfaces.