Synthesis of deep-ultraviolet (DUV) transparent materials, especially those with noncentrosymmetric structures, remains a great challenge in the solid-state chemistry community. A new DUV transparent borate, K3B5O8(OH)(2), was discovered with a short absorption edge below 200 nm. The title compound crystallizes in a noncentrosymmetric, polar space group, Fdd2 (point group mm2), with the following cell parameters: a = 13.736(9) angstrom, b = 19.317(12) angstrom, c = 7.606(5) angstrom. The structure of K3B5O8(OH)(2) features a 3D framework composed of [B5O8(OH)(2)](3-) basic building units that are linked by contacts with K+ cations and O-H center dot center dot center dot O hydrogen bonds. Second harmonic generation (SHG) measurements were performed, and an SHG efficiency of 0.5 x SiO2 was observed. Symmetry dictates that the chi(14) component of the macroscopic NLO susceptibility is equal to zero in mm2, which prevents the maximal component of the microscopic NLO susceptibility for [B5O8(OH)(2)](3-) units (chi(14)) from contributing to the macroscopic NLO susceptibility of the crystal and therefore limits the SHG efficiency. In contrast, large SHG effects can be observed from compounds containing [B5O10] units that crystallize in point groups with nonzero chi(14), such as 222. These findings provide insight into understanding the relationship between crystal structure and SHG efficiency in [B5O10]-based compounds and discovering other borate-based DUV materials.