Copper(Cu) is well known as a representative material for viral inactivation, but it easily loses its antiviral activity by self-oxidation. The heterojunction of Cu clusters with TiO₂ (TiO₂-CuO_x) is a promising approach to improve its antiviral performance and stability under visible light illumination. Since the antiviral activity varies depending on the species of Cu, it is important to find the optimal oxidation state of Cu for the application of TiO₂-CuO_x. In this study, the antiviral efficacy and stability of TiO₂-CuO_x synthesized according to Cu oxidation state were evaluated against phix174 (DNA virus) in the absence and presence of visible light illumination. The XRD and XPS spectra revealed that the synthesized TiO₂-CuO_x are composed of zero-valent (Cu⁰), cuprous (Cu₂O), cuprous-cupric balanced (Cu_xO) and cupric (CuO) dominant states, respectively. In dark condition, the antiviral activity by TiO₂-CuO_x increased in the order of TiO₂-CuO < TiO₂-Cu_xO < TiO₂-Cu­₂O < TiO₂-Cu⁰ due to the enhanced oxidative damages of reduced state of Cu. Under visible light illumination, TiO₂-CuO_x exhibited the highest rate of viral inactivation resulting in 0.16 log inactivation/min and maintained its activity for five reuse cycles, while TiO₂-Cu⁰ was faced with a significant decrease in antiviral activity. Various experiments (photo-electrochemical analyses, q-PCR, antiviral inactivation tests under different conditions, etc) suggested that TiO₂-Cu­₂O exhibited higher photocatalytic efficiency resulting in the reduction of ≡Cu(II) to ≡Cu(I) by interfacial charge transfer compared to TiO₂-Cu⁰. In addition, it is suggested that the reduced ≡Cu(I) in TiO₂-Cu­₂O can activate dissolved oxygen for the in situ production of H₂O₂ and subsequently generate high-valent copper species (≡Cu(III)), which has strong virucidal activity. Thus, this work identifies the antiviral properties and reaction mechanisms depending on Cu oxidation state in TiO₂-CuO_x, and proposes TiO₂-Cu­₂O as a promising antiviral material with high performance and reusability under visible light illumination.