The title compound, K[B(5)O(7)(OH)(2)], was obtained from a hydro-thermal reaction. The structure is composed of one K(+) cation and a polyborate (1) (∞)[B(5)O(7)(OH)(2)](-) anion, which consists of two six-membered rings linked by a common BO(4) tetra-hedron. The [B(5)O(7)(OH)(2)](-) units are linked together through two exocyclic O atoms to neighbouring units, forming a helical chain structure extending parallel to [010]. Adjacent chains are further connected into a three-dimensional structure by K-O bonds and weak O-H⋯O hydrogen-bond inter-actions.