BaTiO3–SnO solid solutions have been investigated from the viewpoints of synthesis and Sn ion valence. First-principles calculations show that the solution energy of Sn2+ into the Ba sites in BaTiO3 is less than that into the Ti sites under high pressure. The BaTiO3–SnO solid solutions have been synthesized under high pressure (∼20 GPa) and temperatures using a laser-heated diamond anvil cell. The synthesized materials have been characterized using x-ray diffractometry, scanning transmission electron microscopy, and energy-dispersive x-ray spectroscopy. It is found from these various methods that we have successfully synthesized uniform solid solutions of BaTiO3–SnO. Furthermore, it is also clarified by the Sn L3-edge electron energy loss spectra measurements that the valences of the Sn ions in the BaTiO3–SnO solid solution are 2+. These results indicate that the Sn2+ ions are substituted into the Ba sites, according to the ion size. Consequently, the Sn ions can be substituted into the Ba sites of the shrinking BaTiO3 lattice under high pressure, which is similar to the Ca and Sn co-substitution into Ba sites under ambient pressure as reported previously.