Herein, Ag-decorated CuO@TiO₂ (CuO@TiO₂-Ag) semiconductor heterojunctions are prepared using a porous CuO nanowire (NW) matrix for visible light photocatalysis applications. The CuO NW matrix is synthesized by wet-oxidation-based growth on a commercial Cu plate. Conformal coating of TiO₂ on the CuO NW surfaces is successively achieved by atomic layer deposition, following which Ag nanoparticles (NPs) are deposited to synthesize a Ag-decorated CuO@TiO₂ photocatalyst film. Microstructural and chemical analyses reveal uniform synthesis of a CuO@TiO₂ heterojunction as well as a binding energy shift at the interface to form balanced Fermi energy, respectively. The performance of CuO@TiO₂-Ag as a visible-light photocatalyst is evaluated under 20-W low-power visible-light LED illumination for the photodegradation of methylene blue. In addition, the effects of the TiO₂ layer and the Ag NPs on the decomposition efficiency are investigated. To control the thickness of TiO₂ and the dosage of Ag NPs, the photodegradation of CuO@TiO₂-Ag is optimized at a rate of 0.0133 min⁻¹, which is approximately 3 and 2.5 times higher than those of a bare CuO@TiO₂ heterojunction and Ag-deposited CuO, respectively. The high decomposition efficiency achieved in this study results from the combined effects of the conformal core–shell heterojunction formed by atomic layer deposition and the well-distributed Ag NPs.