In this study, a TiO₂–Cu₂O composite catalyst with enhanced efficiency for the gas-phase degradation of toluene was synthesized using the glucose solvothermal method. The TiO₂–Cu₂O composite catalyst exhibited enhanced stability compared to individual materials (TiO₂ or Cu₂O). Characterization results revealed the formation of a p–n heterojunction between TiO₂ and Cu₂O; therefore, the toluene degradation rate is promoted. For the degradation experiments with toluene, the TiO₂–Cu₂O composite exhibited enhanced efficiency (63%) for toluene degradation compared to TiO₂ (40%) under simulated visible-light irradiation. Furthermore, the degradation rate of gas-phase toluene (171.6 mg m⁻³) by TiO₂–Cu₂O reached 79% compared to Cu₂O (55%) under simulated sunlight irradiation. A mechanistic study indicated that the heterojunction improves the catalytic capability. The conduction band of TiO₂ is situated below that of Cu₂O; thus, the electron–hole transfer between the heterojunction is thermodynamically permitted. The photogenerated electrons can move from Cu₂O to TiO₂, while holes migrate from the valence band of TiO₂ to Cu₂O, thereby preventing the recombination of electrons and holes. hence, the photocatalytic activity is improved.