Trimethylammonium bis(citrato)borate monohydrate (CH3)3NH[(C6H6O7)2B]•H2O (I) was synthesized for the first time. Its crystal structure was solved from single crystal X-ray diffraction data (C15H24NO15B: a = 9.7795(3) Å, b = 10.7367(3) Å, c = 10.9245(4) Å, = 84.883(1)º, = 77.341(1)º, = 69.126(1)º, V = 1045.7(1) Å3, Z = 2, space group P-1, 4888 reflections with Rint = 0.0230; R1 = 0.058, T = 173(2), ρcalc = 1.490 g/cm3, ρexp = 1.510 g/cm3). The structural units of crystal I (the large bis(citrato)borate anion with a spirane structure, the trimethylammonium cation, and the crystal water molecule) form a layered packing with a system of six O─H...O, (OI) and N─H...O, (OI) type hydrogen bonds. In complex anion [(C6H6O7)2B]- two citric acid molecules are bidentately coordinated to the boron atom via the O atoms of central carboxyl and α-hydroxyl groups. The complex anion has pseudosymmetry C2. The XRD data were compared with crystal data for complexes of boron and citric acids and alkyl-substituted cations studied previously.