The use of hematite as a photoanode has been limited due to its optoelectronic properties and a large overpotential for photo-assisted water splitting. In this study, a cobalt-phosphate (Co–Pi) layer was deposited on hematite photoanodes (α-Fe2O3) in order to increase the lifetime of the photogenerated holes and to lower the applied bias. The influence of the Co–Pi catalyst thickness, the pH of deposition, and the scan rate of the photoelectrochemical (PEC) measurements were optimized using the Taguchi statistical method. An L9 orthogonal array, with the three factors of control at three levels, was employed to determine the optimum conditions for water splitting of neutral water with minimum applied bias. After depositing Co‒Pi layer on the α-Fe2O3 at different conditions, the PEC was measured via the linear sweep voltammetry (LSV) and chronoamperometry. The analysis of variance of the mean overpotential in comparison with the bare hematite and the signal-to-noise ratio indicated the great influence of catalyst thickness on the decreasing of overpotential for photo-assisted water splitting in comparison with the bare α-Fe2O3. The characterization of the catalyst and the α-Fe2O3 morphology were carried out by SEM, XRD, and EDS. The optimized photoanode showed an enhancement of around two times for photocurrent and 1.5 times in incident photon-to-current conversion efficiency (IPCE) compared to the bare α-Fe2O3. These findings result in an understanding of the role of the Co–Pi catalyst role in improving the hematite photoanode.