The novel hydrogensquarate salt of 4-cyanopyridine was synthesized, and its structure and properties were elucidated in detail spectroscopically, thermally, and structurally, using single-crystal X-ray diffraction, linear-polarized solid-state IR spectroscopy, UV spectroscopy, TGA, DSC, DTA, and positive and negative ESI MS as well as H-1 and C-13 NMR methods. Quantum chemical methods were used to calculate the electronic structure, vibrational data, and electronic spectra. 4-Cyanopyridinium hydrogensquarate monohydrate crystallizes in the space group P (1) over bar and exhibits a layered structure with molecules linked by intermolecular NH center dot center dot center dot O(HSq-) (2.651 angstrom) and HOH center dot center dot center dot O-(HSq(-)) (2.792 and 2.563 angstrom) hydrogen bonds with participation of cations, anions, and the solvent molecule. The formation of stable layers of the type (2HSq(-).2H(2)O)(n) and the observation of a red color in the solid state is discussed. The optical and magnetic properties were elucidated in comparison to the data for neutral 4-cyanopyridine as well as its four known salts.