In the crystal structure of the title compound, K(2)[B(10)O(14)(OH)(4)]·H(2)O, the polyborate [B(10)O(14)(OH)(4)](2-) anions are linked together through their common O atoms, forming a helical chain-like structure. Adjacent chains are further connected into a three-dimensional structure by O-H⋯O hydrogen bonds. The water mol-ecules and potassium cations are located between these chains. Further O-H⋯O hydrogen bonds occur between the anions and the water mol-ecules.