Physicochemical properties of titanium dioxide nanoparticles (TiO2 NPs) can be tuned by doping with metals or nonmetals. Copper (Cu) doping improved the photocatalytic behavior of TiO2 NPs that can be applied in various fields such as environmental remediation and nanomedicine. However, interaction of Cu-doped TiO2 NPs with human cells is scarce. This study was designed to explore the role of Cu doping in cytotoxic response of TiO2 NPs in human lung epithelial (A549) cells. Characterization data demonstrated the presence of both TiO2 and Cu in Cu-doped TiO2 NPs with high-quality lattice fringes without any distortion. The size of Cu-doped TiO2 NPs (24 nm) was lower than pure TiO2 NPs (30 nm). Biological results showed that both pure and Cu-doped TiO2 NPs induced cytotoxicity and oxidative stress in a dose-dependent manner. Low mitochondrial membrane potential and higher caspase-3 enzyme (apoptotic markers) activity were also observed in A549 cells exposed to pure and Cu-doped TiO2 NPs. We further observed that cytotoxicity caused by Cu-doped TiO2 NPs was higher than pure TiO2 NPs. Moreover, antioxidant N-acetyl cysteine effectively prevented the reactive oxygen species generation, glutathione depletion, and cell viability reduction caused by Cu-doped TiO2 NPs. This is the first report showing that Cu-doped TiO2 NPs induced cytotoxicity and oxidative stress in A549 cells. This study warranted further research to explore the role of Cu doping in toxicity mechanisms of TiO2 NPs.