Crystal structure of high-temperature polymorphic modification, α-CsB₅O₈, was determined from single-crystal X-ray diffraction data. The phase crystallizes in space group P2₁/c with a = 7.122(2), b = 9.640(3), c = 11.411(3) Å, β = 116.64(2)° and V = 700.3(3) ų, Z = 4. It contains new zigzag layer polyanions built up from rigid [B₅O₈]^- pentaborate groups that consist of four triangles and a tetrahedron condensed to a double ring via common tetrahedron. The cesium cations are located in large cavities of the layers and they are coordinated by nine oxygen atoms with distances from 3.037 to 3.429 Å. Five of the oxygen atoms are placed in the same layer as cesium atom; other four ones are distributed between two adjacent layers. Under heating this structure demonstrates a highly anisotropic thermal expansion (α₁₁ = 27, α₂₂ = 61, α₃₃ = -8 × 10^-6 K^-1); the thermal expansion in the direction perpendicular to the layer is the intermediate one.