The non-stoichiometric chromium selenide Cr5.095(5)Se8 was prepared under high-pressure hightemperature conditions. The structure was refined from X-ray powder data with the Rietveld method in the non-conventional monoclinic space group F2/m, a = 12.2992(2), b = 7.12753(12), c = 11.4486(2) Å , β = 90.927(1)° and V = 1003.49(3) Å3. Three of the four unique Cr sites are fully occupied, and one site is only partially occupied. The structure may be viewed as being composed of alternating full and metal deficient layers which are oriented perpendicular to the crystallographic c axis of the pseudo-hexagonal unit cell. All Cr atoms are in an octahedral environment of six Se atoms. The CrSe6 octahedra of neighbouring layers share common faces whereas the octahedra with layers are joined by common edges. As a result short Cr-Cr distances of 2.867(3) and 2.951(6) Å are found across common faces while Cr-Cr separations between CrSe6 octahedra sharing edges are significantly longer. From a formal point of view charge balance requires a formulation as Cr4+0.715Cr3+4.38Se8. On the basis of the distortion of the CrSe6 octahedra the Cr4+ ions are mostly located on two different sites.