In single crystals of a new monoclinic (C2/m) form of tricopper(II) diselenium(IV) dichloride hexaoxide, Cu3(SeO3)2Cl2, the Se atom is in the 4i position, while the two Cu atoms are in 2a and 4i positions. The structure is based on layers of CuO4Cl trigonal bipyramids, CuO4 square planes and SeO3E tetrahedra. The Cu polyhedra are connected by edge- and corner-sharing to form [010] chains and these chains are bridged by the Se atoms to form (001) layers. The compound is isostructural with Cu3(TeO3)2Br2.