A new alkali metal borate with formula Na6 [B4O5 (OH)4]3. 8H2O single crystal has been grown by slow evaporation method under room temperature condition for the first time. The structural analysis reveals that the grown crystal crystallizes in the rhombohedral space group R32 with cell parameters a = 11.27 Å, b = 11.27 Å, c = 21.27 Å and α = β = 90°, γ = 120°. The intermolecular interactions of symmetric stretching of BO3 and asymmetric stretching of BO4 were identified by Fourier-transform infrared spectroscopy (FT-IR). Optical investigation on the hydrated borate shows that the material possessed a high transmittance of about 60% in the entire UV- visible region with an optical absorption edge at 220 nm and the band gap was found to be 4.972 eV. The theoretical calculations were performed in order to find the linear optical constant such as linear absorption coefficient, extinction coefficient and reflectance etc. Thermogravimetric (TG)/Differential scanning calorimetry (DSC) analysis for the grown crystal displayed that the material is stable up to 116 °C and having the melting point of 145 °C. The frequency independent dielectric constant and dielectric loss were performed at different temperature. AC conductivity was studied by means of electrical relaxation process and it is due to the presence of water molecules in the compound. The third order nonlinear susceptibility (χ3), nonlinear refractive index (n2) and the absorption coefficient (β) are found to be in the order of 1.36 × 10−6 esu, 3.88 × 10−8 cm2/W and 0.026 × 10−4 cm/W respectively. The strong optical limiting behaviour of the material could be attributed to the two photon absorption mechanism and negative nonlinearity due to self-defocusing effect. © 2017 Elsevier B.V.