In the tetraborate anion of the title compound, K2[B4O5(OH)4]·H2O, the bridging B—O bond lengths of the tetrahedral BO4 and the trigonal-planar BO3 units are slightly longer than the corresponding terminal B—OH bond lengths. The crystal structure is stabilized by intermolecular O—H...O, O—H...Owater and Owater—H...O hydrogen bonds, generating a three-dimensional network. The two potassium cations both show a coordination number of 9.
