We report on the first synthesis of a potassium orthoselenate(VI), K-6(SeO4)(SeO5), and the structure determination from synchrotron powder diffraction data. The title compound crystallizes in the tetragonal space group P4(1)2(1)2 with a = 8.1259(1) angstrom, c = 17.4953(2) angstrom, V = 1155.21(2) angstrom(3), and Z = 4. Selenium displays two different complex anions, tetrahedral SeO42- and trigonal-bipyramidal SeO54-. When the formula is reduced to A(3)B, the spatial arrangement of the constituting building units can be derived from the Li3Bi type of structure.