The organic-inorganic hybrid nonlinear optical (NLO) material NH4B(d-(+)-C4H4O5)2·H2O (NBC) was synthesized in a borate-carboxylic acid system. Its structure was determined by single crystal X-ray diffraction. It crystallizes in the orthorhombic system, space group Pna21 (No. 33), with cell parameters a = 11.484(6) Å, b = 5.354(3) Å, c = 21.079(12) Å, V = 1296.0(12), Z = 4. It exhibits a three-dimensional pseudo tunnel structure consisting of fundamental building block [B(d-(+)-C4H4O5)2]– anions. The small cavities are occupied by the H2O molecules and NH4+ cations, which stabilize the whole structure by O–H···O and N–H···O hydrogen bonds. The powder X-ray diffraction (PXRD) of the crystal was also recorded. Elemental analyses, FT-IR and FT-Raman spectra analyses, thermal analysis, and diffuse-reflectance spectra for the compound are also presented, as are band structures and density of states calculation. Nonlinear optical measurements indicate that the material has second harmonic generation (SHG) properties and is phase-matchable.