The crystallization of a new allotrope of cyclo-octaselenium, named γ-monoclinic selenium, from a solution of dipiperidinotetraselane in carbon disulphide is described. The allotrope γ-Se₈ crystallizes in space group P2₁/c(no. 14) with a= 15.018(1), b= 14.713(1), c= 8.789(1)Å, β= 93.61(1)°, and z= 64 (atoms). The crystal structure has been determined by X-ray diffraction from Mo-K_α diffractometer data and refined to R 0.047 for 2 525 observed reflections. There are two crown-shaped Se₈ rings in the asymmetric unit, with bond lengths, bond angles, and dihedral angles in the ranges 2.326(3)–2.344(3)Å, 103.3(1)–109.1(1)°, and 96.5–107.2° respectively. The overall averages, 2.334 ± 0.005 Å, 105.8 ± 1.4°, and 101.1 ± 2.2°, are the same, within error limits, as in α- and β-monoclinic selenium. There are more short contacts between the rings than in the α and β forms, the shortest ones being 3.346(3) and 3.404(3)Å.