Rb-4B4O5(OH)(4)2 center dot 3H(2)O has been grown by slow evaporation method with crystal sizes up to 11 mm x 7 mm x 4 mm. Its crystal structure has been determined by single-crystal X-ray diffraction and further characterized by powder X-ray diffraction (XRD). IR, and TGA. It crystallizes in the orthorhombic space group Pbcn, with a = 16.3984(14) angstrom, b = 7.7850(7) angstrom, c = 16.0763(16) angstrom, and Z = 4. It contains isolated B4O5(OH)(4)(2-) polyanions, free water molecules and Rb cations. The Rb(1) and Rb(2) atoms are coordinated by nine and ten oxygen atoms, respectively. Connections between isolated B4O5(OH)(4)(2-) polyanions are provided by Rb polyhedra and the hydrogen-bonding to form a three-dimensional network.