Pure single crystals of Lewis acid-base salt, hydrazinium hydrogen phthalate monohydrate (HHPMH) has been synthesized, crystallized and isolated as a single crystal by slow evaporation method at room temperature (22 ºC). Analysis of analytical, spectral, thermal and structural data confirmed the salt formation in aqueous medium. The analytical data is in favour of the proposed composition HOOC-C6H4COO N2H5·H2O. The compound crystallized in a monoclinic crystal system and P21/C space group with cell parameters, a = 15.409(3) Å, b = 10.292(2) Å and c = 6.7087(12) Å. Though strong intramolecular hydrogen bonding interaction is anticipated between acidic hydrogen of HHPMH and deprotonated carboxylate oxygen surprisingly in the present case it is absent and a weak interaction is observed between acidic hydrogen and water molecule, which is quite evident from the ORTEP diagram. The nature of interactions between hydrazine cation and hydrogen phthalate ion and the crystal packing diagram from various types of intermolecular contacts were determined by Hirshfeld surface analysis. The compound was stable up to 40 ºC and undergoes dehydration from 40-70 ºC with an endotherm at 47 ºC. The experimental and theoretical infrared spectra are also in favour of the formation of ionic crystal. The molecular structure was optimized using B3LYP method by 6-311G (d,p) basis set. The energy gap between HOMO and LUMO is 7.7340 eV. Theoretical calculation based on DFT studies and natural bond orbital analyses has been carried out to evaluate the molecular electrostatic potential. The structure was optimized by DFT and the vibrational frequencies were calculated. Comparison of experimental and optimized structures revealed that there is a good coincidence between the two.