The crystal structure of Rb₅B₁₉O₃, monoclinic, sp. gr. C2lc, a = 18.211(3), b = 11.519(2), c = 13.310(2) Å, β= 95.63(1)°, V = 2778(1) ų, Z = 4, d_calc = 2.714 g/cm³. It is determined by X-ray diffraction analysis (P2₁ Syntex diffractometer, MoKα radiation, 2280 reflections, R = 0.050) starting from the atomic coordinates for the isotructural K₅B₁₉O₃₁. The thermal vibrations of Rb and O atoms are considered in the anisotropic approximation. The structure is a three-dimensional framework built of complicated anions of four types: triangles; tetrahedra; [B^IVB^III ₂O₅]^- ringsconsisting of two triangles and a tetrahedron; and double [B^IVB^III ₄O₈]^- rings consisting of two simple rings with sharing a tetrahedron. Alternating ordinary and double rings form helical chains about the 2₁ screw axes. Contraction of these chains during heating is presumably the main reason for a strong anisotropy of thermal strain revealed in the crystals by high-temperature X-ray diffraction method.