Nanocrystalline porous tin dioxide (SnO2) materials have been obtained employing room temperature ionic liquids (1-hexadecyl-3-methylimidazolium bromide, C16MimBr) as a template via a green sol−gel method at ambient temperature followed by a suitable thermal treatment. These materials have been thoroughly characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, FTIR, and nitrogen adsorption−desorption. A careful tuning of heat-treatment procedures allowed the preparation of SnO2 functional materials with Brunauer−Emmett−Teller surface areas ranging from 38 to 140 m2 g−1, an average pore size between super-micropore (1−2 nm) and mesopore (10 nm) range, and a mean particle size from 3.0 to 10.0 nm. The applications in gas sensors for the nanostructures reveal that the obtained SnO2 materials exhibit highly sensitive, fast-responding, reproducible, and size selective sensing behaviors. The sensor characteristics were discussed in relation to the architectures of the materials, which disclose that the gas-sensor properties are strongly structure-dependent.