<p>A novel nonlinear optical organic single-crystal, 4-aminobenzoate p-methylbenzene sulphonate monohydrate (4APMSM) with a molecular formula C₁₄H₁₇NO₆S has been successfully synthesized through a facile slow evaporation technique from ethanol-water solvent mixture at ambient temperature. The crystallographic architecture of the synthesized 4APMSM novel adduct was unravelled via single-crystal X-ray diffraction, confirming its structural integrity and lattice parameters. The 3D Hirshfeld surface analysis and 2D fingerprints visualize and quantify intermolecular interactions within the grown crystal. Fourier transform infrared spectroscopy provides insights into the vibrational modes associated with functional moieties and evidence the incorporation of lattice water molecules. The optical characteristics were systematically investigated employing UV–Vis absorption and steady state fluorescence spectroscopy, which delineate the electronic transitions and luminescent behaviour of the title compound. The UV–Vis spectrum revealed a cut-off wavelength of 278 nm, corresponding to an optical band gap of 4.1 eV, affirming 4APMSM’s low band gap nature and its potential for strong optical nonlinearity and efficient charge transfer. Thermal stability was quantitatively examined using thermogravimetric analysis (TGA) coupled with differential thermal analysis (DTA), revealing decomposition thresholds and thermal events, with stability observed up to 183 °C. Mechanical robustness was assessed via Vickers microhardness tester, quantifying its resistance to localized deformation. Frequency-dependent dielectric measurements were carried out over a range of temperatures to study the dielectric response of the title compound. Moreover, third-order nonlinear optical (NLO) susceptibility of 4.108 × 10⁻¹² esu was probed through Z-scan technique, revealing the crystal’s potential for advanced photonic and optoelectronic functionalities.</p>