The crystal structure of parageorgbokiite, β-Cu₅O₂(SeO₃)2Cl₂, a new mineral species from the fumaroles of the Great Fissure Tolbachik eruption, in the Kamchatka Peninsula, Russia, has been solved from single-crystal X-ray-diffraction data by direct methods and refined to R₁ = 0.032 for 1388 unique reflections with F₀ ≥ 4σ_F. The mineral is monoclinic, space group P2₁/c, a 5.3982(5), b 8.0543(8), c 11.1277(10) Å, β 99.258(2)°, V 477.52(8) ų, Z = 2. The structure contains three symmetrically unique Cu positions, each octahedrally coordinated by O^2- and Cl^- anions. There is one symmetrically independent Se⁴⁺ cation coordinated by three O atoms, to form SeO₃ trigonal pyramids. The structure of parageorgbokiite is best described in terms of arrangements of the strong M-O bonds (M = Se⁴⁺ and Cu²⁺). The O1 atom is not bonded to Se⁴⁺ cations and is tetrahedrally coordinated by four Cu²⁺ cations, which results in the formation of an oxocentered O1Cu₄ tetrahedron. Two O1Cu₄ tetrahedra share a Cu3...Cu3 edge to form O₂Cu₆ dimers that are further linked by sharing the Cu2 corners to form O₂Cu₅ infinite chains parallel to the a axis. The SeO₃ pyramids are attached to the faces of the oxocentered tetrahedra in a "face-to-face" position to form complex ([O₂Cu₅] [SeO₃]₂)²⁺ chains and a three-dimensional framework with channels occupied by the Cl^- anions. A comparison of parageorgbokiite and georbokiite shows that the two minerals have the same space group P2₁/c (monoclinic), but differ in their unit-cell parameters. The structures of both minerals are based upon the same complex ([O₂Cu₅] [SeO₃]₂)²⁺ chains. However, the arrangements of the chains in the structures are different. The channels in parageorgbokiite are larger than those in georgbokiite. From the lower density of parageorgbokiite in comparison to georbokiite, we suggest that parageorgbokiite is a high-temperature modification of Cu₅O₂(SeO₃)₂Cl₂ that is either chemically or kinetically stabilized at ambient conditions.