A potential ultraviolet (UV) nonlinear optical (NLO) material, K3B3O4(OH)(4)center dot 2H(2)O, was successfully synthesized by hydrothermal methods. The compound crystallizes into the Cmc2(1) space group and exhibits isolated [B3O4(OH)(4)](3-) anion groups connected by O-H center dot center dot center dot O hydrogen bonds. The UV vis diffuse reflectance spectrum shows that K3B3O4(OH)(4)center dot 2H(2)O has a wide transparency range with an absorption edge below 190 nm. Powder second harmonic generation (SHG) measurements using 1064 nm radiation revealed a moderate efficiency, 0.8 X KDP. Additional particle size vs SHG efficiency measurements indicate that K3B3O4(OH)(4)center dot 2H(2)O is type I phase-matchable. Our calculated results show that the borate groups as well as the waters of hydration determine the NLO properties of K3B3O4(OH)(4)center dot 2H(2)O.