Efficient photoelectrochemical (PEC) water splitting requires rapid separation and transport of photogenerated carriers. These processes can be enhanced by engineering a built-in electric field (BIEF) and tuning the electronic structure of the photoanode. In this study, we introduce a Ni-based metal–organic framework (Ni-MOF) as a surface cocatalyst on hematite (α-Fe₂O₃) to form an interfacial heterojunction that generates a strong BIEF. Incorporation of Fe into the Ni-MOF creates Ni–O–Fe bonds, while –OH functional groups in the MOF ligands donate electrons and modulate the electronic structure of α-Fe₂O₃/NiFe-MOF(2OH). The resulting photoanode achieves a photocurrent density of 3.48 mA·cm⁻² at 1.23 V vs. reversible hydrogen electrode (V_RHE), approximately four times that of pristine α-Fe₂O₃ (0.88 mA·cm⁻²). Mechanistic studies reveal that the strong BIEF, vigorous Ni–O–Fe coupling, and electron-donating –OH groups synergistically facilitate electron delocalization, suppress charge recombination, prolong hole lifetime, and improve adsorption of oxygen evolution intermediates. These effects significantly enhance carrier separation and transfer efficiency, leading to superior PEC water-splitting performance.