Using Rietveld refinement the crystal structure of the β-rubidium pentaborate has been found to be isotypic with β-potassium pentaborate: (61) Pbca - (c)¹⁴, oP112, a = 7.550(1) Å, b = 11.842(1) Å, c = 14,805(1) Å, V = 1323.7(2) ų, Z = 8, D_calc = 2.68 · 10³kg/m³. With the aid of high temperature X-ray diffractometry a strongly anisotropic thermal expansion has been observed: α_a = 61·10^-6 K^-1, α_b = 23 · 10^-6K^-1 and α_c = 4.7 · 10^-6K^-1. This anisotropy may be caused by anisotropic thermal vibrations of heavy atoms as rubidium.