In this study, visible-light-responsive Au nanostar-decorated Cu₂O/TiO₂ ternary nanocomposites were successfully synthesized and evaluated for the photocatalytic degradation of methylene blue (MB). The incorporation of Au nanostars introduced a strong localized surface plasmon resonance (LSPR) effect, enhancing visible-light absorption and promoting photogenerated charge carrier generation. Simultaneously, the Cu₂O/TiO₂ heterojunction, with an estimated band gap of approximately 2.3 eV, facilitated efficient charge separation and extended the optical response into the visible region, as confirmed by diffuse reflectance spectroscopy (DRS). Dynamic light scattering (DLS), zeta potential, and X-ray diffraction (XRD) analyses confirmed the formation of a well-dispersed, crystalline, and colloidally stable ternary nanocomposite. The enhanced photocatalytic activity was attributed to the synergistic effects of plasmon-induced light harvesting, efficient interfacial charge transfer, and suppressed electron–hole recombination, leading to increased generation of reactive oxygen species (•OH and O₂•⁻). The effects of Au nanostar loading and photocatalyst dosage were systematically investigated, revealing optimum performance at intermediate loading levels, while excessive loading reduced efficiency because of active-site shielding and light scattering. Under optimized conditions, the photocatalyst achieved approximately 97% degradation of methylene blue within 90 min under visible-light irradiation and retained approximately 84% of its initial activity after ten reuse cycles. These results demonstrate the potential of the Au nanostar/Cu₂O/TiO₂ nanocomposite as an efficient and reusable visible-light-driven photocatalyst for wastewater treatment.