The high-pressure perovskite K2/3Th1/3TiO3 was synthesized at P = 6 GPa and T = 1200° C. This compound does not form at ambient pressures, as both solid-state reaction and synthesis from the melt yield a mixture of thorianite (ThO2) and jeppeite (K2Ti6O13). K2/3Th1/3TiO3 is a partially ordered derivative of the ideal perovskite structure, which crystallizes with tetragonal symmetry, in space group P4/mmm, a = 3.9007(2), c = 7.8099(7) Å, V = 118.83(2) Å3, Z = 2. The structure of this compound was refined by the Rietveld method from the X-ray diffraction powder data. The degree of disorder calculated from the refined cation occupancies of the 1a and 1b sites is 58%. The K1+ cations preferentially enter the 1a site, whereas most Th4+ is accommodated in the comparatively smaller 1b site (polyhedral volumes are 53 and 46 Å3, respectively). In response to this two-dimensional (planar) ordering, the Ti4+ cations are displaced by about 0.1 Å toward the planes populated by the lower-charged cations. K2/3Th1/3TiO3 and related structures may be a viable repository for Th in Ti-rich alkali metasomatites in the lithospheric upper mantle.