An ordered mesoporous In2O3 material with crystalline walls has been synthesized through the nanocasting method. It was investigated as an impedance-type humidity sensor for the detection of water vapor. A nanostructured In2O3 matrix has been obtained by the hard template route from the KIT-6 silica template. The crystalline In2O3 belonged to the Ia3d space group, and its structure was characterized by X-ray diffraction (XRD), N2 adsorption–adsorption, and transmission electron microscopy (TEM). The sensor based on mesoporous In2O3 showed excellent performance in terms of humidity changes and favorable stability. Through the analysis of its semiconductor characteristics, Kelvin equation, and complex impedance, we found that the 3D mesoporous structure contributes greatly to the improvement of humidity-sensitive properties. A possible mechanism was established to explain the excellent performance of the mesoporous In2O3-made humidity-sensing device. The evaluation of its electrical characterization and the establishment of its sensing mechanism shows that mesoporous In2O3 is a good candidate for developing humidity sensors. It has potential applications in the chemical-sensing field.