Cu3(SeO4)2(SeO3OH)2(H2O)16 (1) is a novel copper selenate hydrate and the first inorganic compound to contain linear octahedral–tetrahedral heptamers. The heptamers are formed by successive polymerization of two SeO3OH tetrahedra, two SeO4 tetrahedra, and three Cu(H2O)4O2 octahedra. The linkages between the octahedra and tetrahedra are in the trans-configuration, which is in agreement with the Jahn–Teller distortion of the octahedral coordination geometry of Cu2+.This work was supported by the Russian Science Foundation (grant 14-17-00071). X-ray diffraction studies have been carried out at the XRD Centre of Saint-Petersburg State University.