A new ultraviolet (UV) nonlinear optical (NLO) crystal Sr4B 10O18(OH)2·2H2O has been synthesized using the hydrothermal method. It crystallizes in the triclinic space group P1 (no. 1) with lattice constants a = 6.4991(6) A˚, b = 6.5059(6) A˚, c = 11.3175(11) A˚, α = 88.458(6)°, β = 86.019(5)°, γ = 61.351(5)°, Z = 1. The fundamental building block of the structure is a [B10O23]^16- unit which consists of two stereo-isomeric configurations of [B5O 12]^9- groups. The structure exhibits a ∞[B10O18]^6- three dimensional network with 9-membered tunnels along the b-axis where the Sr cation, OH anion and H2O molecule are located. Powder second harmonic generation (SHG) measurements indicate that Sr4B10O18(OH) 2·2H2O is type-I phase-matchable and displays a SHG response of about 2 × KH2PO4. The UV-Vis-NIR diffuse reflectance spectrum shows that its cutoff edge is lower than 190 nm. Density functional theory (DFT) calculations on electronic structures and SHG coefficients confirm the experimental results.