Several series of Mn based multilayers (Mn/SnO2,Mn/Si, and Mn/SiO2)N were prepared by magnetron sputtering on Si(100) substrates at room temperature (RT). A ferromagnetic phase with TC above 300 K is detected only for Mn/SnO2. The highest ferromagnetic signal corresponds to the presence of Mn in a mixed valence state combined with the presence of tin oxide. Grazing incidence high-angle x-ray diffraction revealed the formation of MnO and SnO1+x nanocrystals. After annealing, no traces of metallic Mn or MnO are detected from x-ray absorption near-edge structure (XANES) or diffraction; Mn2O3 and Mn3O4 oxides and polycrystalline SnO2 were identified. A clear ferromagnetic signal below 42 K with high coercive field is then detected corresponding to Mn3O4 but the RT ferromagnetic component decreases drastically. No mixed Mn–Sn–O phases are detected in as-grown or annealed samples. The close vicinity of MnO1+x and SnO1+x nanograins at the multilayer interfaces seems to be the condition necessary for the RT ferromagnetism.