A novel hydrated cesium hydroxypentaborate is synthesized from boric acid flux at 180°С, and its crystal structure is determined by X-ray diffraction. The compound crystallizes in the triclinic system, sp. gr. P $$\bar {1}$$ , with the parameters a = 7.8107(5) A, b = 9.1929(8) A, c = 12.3553(11) A, α = 98.98(1)°, β = 106.32(1)°, γ = 91.10(1)°, V = 839.1(1) A3, Z = 4. The structure is refined to R1 = 5.8% with anisotropic displacement parameters for all nonhydrogen atoms, with allowance for the non-merohedral twinning by a twofold axis coinciding with the b axis. The twin components were 0.546(1) and 0.454(1). The features of the hydrogen bonds in Cs[B5O7(OH)2] · 0.5H2O are analyzed in comparison with isostructural rubidium analogue. The unit-cell parameters a and c of A[B5O7(OH)2] · 0.5H2O pentaborates are found to systematically increase with an increase in the radius of A cation in the NH4 → Rb → Cs series; the small variation in the b parameter is due to the rigidity of the pentaborate chains elongated along this direction.