Investigation on the formation mechanism of the β-NiS@Ni(OH)2 nanocomposite electrode for electrochemical water splitting application was attempted with the use of the hydrothermal processing technique. Formation of single-phase β-NiS, Ni(OH)2 and composite-phase β-NiS@Ni(OH)2 has been thoroughly analyzed by X-ray diffractometer (XRD) spectra. Three different kinds of morphologies such as rock-like agglomerated nanoparticles, uniformly stacked nanogills, and uniform nanoplates for β-NiS, Ni(OH)2, and β-NiS@Ni(OH)2 materials, respectively, were confirmed by SEM images. The characteristic vibration modes of β-NiS, Ni(OH)2, and β-NiS@Ni(OH)2 nanocomposites were confirmed from Raman and Fourier transform infrared spectra. Near band edge emission and intrinsic vacancies present in the nanocomposites were retrieved by photoluminescence spectra. The optical band gaps of the synthesized nanocomposites were calculated as 2.1, 2.5, and 2.2 eV for β-NiS, Ni(OH)2, and β-NiS@Ni(OH)2 products, respectively. The high-performance electrochemical water splitting was achieved for the β-NiS@Ni(OH)2 nanocomposite as 240 mA/g at 10 mV/s from a linear sweep voltammogram study. The faster charge mobile mechanism of the same electrode was confirmed by electrochemical impedance spectra and a Tafel slope value of 53 mV/dec. The 18 h of stability was achieved with 95% retention, which was also reported for the NiS@Ni(OH)2 nanocomposite for continuous electrochemical water splitting applications.