The new ternary mixed-valent copper selenide KsCusSes has been prepared by the reaction of elemental copper with KzSes and additional selenium (molar ratio 1 : 3 : 1) at 623 K. The compound crystallizes in the monoclinic space group C2/m and is isotypic to K&u&&, RbCu&, Rb3CusSes and Cs3CusSeh. The structure consists of L[Cu$e,] layers which are separated by the potassium cations. The copper atoms are &her tetrahedrally distorted or trigonal planar coordinated by the selenium atoms. In the crystal stmcture the trigonal planar coordinated Cu atoms form double chains which are running parallel to the b-axis. These double chains are connected into layers via the distorted CuSe, tetrahedra which share edges and comers with the Se3 triangles around Cu. Iu this part of the structure a very short Cu-Cu distances of only 2.497 (2) A appears which is shorter than the Cu-Cu distance in elemental copper. Because the tetrahedrally coordinated copper ions exhibit unsusual large anisotropic displacement parameters at room temperature, a low-temperature structure determination was performed. At low temperatures the anisotropic displacement coefficients are significantly smaller for all atoms, but even at 150 K some coefficients for the tetrahedrally coordinated Cu atoms are twice as large as for the trigonal planar coordinated Cu centers. However, the experiment shows that the nature of the disorder seems to be predominantely due to an enlarged mobility of the copper ions at room-temperature which is freezed on cooling. Our give no hints for the formation of a superstructure like in K3CugS6.