Transition metal (Co, Fe, Mn)-doped In2O3−y mesoporous oxides are synthesized by nanocasting using mesoporous silica as hard templates. 3D ordered mesoporous replicas are obtained after silica removal in the case of the In-Co and In-Fe oxide powders. During the conversion of metal nitrates into the target mixed oxides, Co, Fe, and Mn ions enter the lattice of the In2O3 bixbyite phase via isovalent or heterovalent cation substitution, leading to a reduction in the cell parameter. In turn, non-negligible amounts of oxygen vacancies are also present, as evidenced from Rietveld refinements of the X-ray diffraction patterns. In addition to (In1−xTMx)2O3−y, minor amounts of Co3O4, α-Fe2O3, and MnxOy phases are also detected, which originate from the remaining TM cations not forming part of the bixbyite lattice. The resulting TM-doped In2O3−y mesoporous materials show a ferromagnetic response at room temperature, superimposed on a paramagnetic background. Conversely, undoped In2O3−y exhibits a mixed diamagnetic-ferromagnetic behavior with much smaller magnetization. The influence of the oxygen vacancies and the doping elements on the magnetic properties of these materials is discussed. Due to their 3D mesostructural geometrical arrangement and their room-temperature ferromagnetic behavior, mesoporous oxide-diluted magnetic semiconductors may become smart materials for the implementation of advanced components in spintronic nanodevices.