<p>The photoelectrochemical cathodic protection (PCP) performance of pure TiO₂ photoanode is significantly limited by its narrow light absorption range, rapid recombination of photogenerated electron–hole pairs, and inefficient electron transfer. A novel TiO₂/RGO (reduced graphene oxide)/CuInS₂ (T/G/Cu) photoanode based on TiO₂ nanotube arrays modified with RGO and CuInS₂ nanoparticles was developed to improve the corrosion resistance of HRB400 rebars. This ternary photoanode featuring heterojunction structures was synthesized through a combination of anodization, potentiostatic deposition, and successive ionic layer adsorption reaction. Comprehensive characterization techniques, including scanning electron microscope, transmission electron microscope, X-ray diffraction, and X-ray photoelectron spectroscopy, confirmed the uniform distribution of layered RGO and CuInS₂ nanoparticles on the surface of TiO₂ nanotubes, indicating the successful formation of n–n type heterostructures. Photoelectrochemical analyses, such as ultraviolet–visible diffuse reflectance spectrum, photoluminescence spectroscopy, electrochemical impedance spectroscopy, and Mott–Schottky, revealed that T/G/Cu n–n heterojunction films exhibited a reduced bandgap (2.87&#xa0;eV), significantly suppressed photogenerated electron–hole recombination, and enhanced the charge transfer efficiency. Under visible light irradiation, T/G/Cu-2 exhibited a cathodic shift in potential to −0.78&#xa0;V <i>vs.</i> SCE (saturated calomel electrode) and generated a photocurrent density of 20.0&#xa0;μA/cm², demonstrating significantly enhanced PCP performance compared to the pure TiO₂ photoanode. The high electrical conductivity and large specific surface area of RGO, along with the built-in electric field within n–n heterostructure, enabling efficient electron migration and cathodic polarization of HRB400 rebar.</p>