The thickness of the photoanode film plays a crucial role in the photoelectrochemical (PEC) water splitting process, significantly impacting the photoconversion efficiency. The optimal thickness of the photoanode enhances the photocurrent density by making a balance between light absorption and electrolyte mass transfer. The main goal of this study was to investigate the effect of thickness on the photoelectrochemical performance of the Al2TiO5-based nanocomposite layer as a photoanode under the sunlight simulator. Al2TiO5/TiO2/Al2O3 nanocomposite was initially synthesized utilizing the citrate sol-gel technique, and after the calcination at 800°C, it was deposited on the fluorine-doped tin oxide coated glass slide (FTO) substrate by the Dr. Blade method in which, several films of various thicknesses can be prepared from a unique paste, by just changing the adhesive tape layers. The Diffuse Reflectance spectroscopy (DRS) technique was utilized to determine the band gap energy value of all specimens, and the field emission scanning electron microscopy (FE-SEM) method was used to investigate the morphology and to determine the thickness of the deposited films. The X-ray diffraction (XRD) analysis exhibited the presence of the tialite (Al2TiO‌5) phase besides the anatase, rutile, and corundum phases. The cyclic voltammetry (CV) measurements were utilized to estimate the accessible surface area in specimens. Different photoelectrochemical results were expected to be obtained by the photoanodes with different thicknesses of the Al2TiO5/TiO2/Al2O3 nanocomposite films. Photoelectrochemical (PEC) measurements revealed that the highest photoconversion efficiency was achievable by the tialite-based sample by the thickness of about 5 µm which was about 20% higher than the obtained value by the thinner or even thicker tialite-based nanocomposite samples.