NiO@rGO nanohybrids have received copious attention in nanomedicine, energy, and environmental research because of their superior physicochemical properties compared to either of their components alone. The increasing applications of NiO@rGO nanohybrids have elicited concerns regarding human and environmental health risks. A lot of research has been done on the toxicity of simple forms of nanoparticles (e.g., NiO or rGO only), but limited work has been done about the toxicological effects of hybrid or composite forms of such nanoscale materials. Here, we designed this research to examine cytotoxicity, oxidative stress, and apoptosis responses of hydrothermally synthesized NiO@rGO nanohybrids on normal rat kidney cells (NRK-52E). The XRD, TEM, SEM, and EDS characterization data confirm the preparation of excellent-quality NiO@RGO nanohybrids of about 24-28 nm sizes with high purity, where crystalline NiO nanoparticles were tightly anchored on RGO sheets. The MTT, trypan blue, and morphology data showed that NiO@rGO nanohybrids induce cytotoxic effects in NRK-52E cells in an approach that was reliant on time and dosage. It was also observed that NiO@rGO nanohybrids generated oxidative stress in NRK-52E cells, as shown by increased reactive oxygen species (ROS) and decreased glutathione levels. The NiO@rGO nanohybrids further induced apoptosis in NRK-52E cells, obvious by chromosome condensation, caspase-3 activation, apoptotic bodies generation, and cell cycle arrest. Besides, NiO@rGO nanohybrids generate significant cytotoxicity and ROS elevation in human lung cancer cells (A549) and human umbilical vein endothelial cells (HUVECs), suggesting that the cytotoxicity of NiO@rGO nanohybrids was not cell-specific. Overall, present data suggested that NiO@rGO nanohybrids induced cytotoxicity in mammalian cell lines through the ROS and apoptosis pathways. This work warrants further in-depth in vivo toxicological investigation of NiO@rGO nanohybrids before their applications in the environment, energy, and biomedicine.