Neonicotinoids are a relatively new type of insecticide to control a variety of pests. Although they are generally considered to be safe, they can lead to harmful effects on human and environmental health. We aimed to investigate possible effects of common neonicotinoid insecticides (acetamiprid, clothianidin, imidacloprid, thiacloprid, and thiamethoxam) on cytotoxicity and DNA damage in human neuroblastoma (SH-SY5Y) and human hepatocellular carcinoma (HepG2) cells. Our results indicated that 50% of inhibitory concentration values of neonicotinoids are in the range of 0.96 to >4 mM in SH-SY5Y cells and 0.53 to >4 mM in HepG2 cells by the methyl tetrazolium and neutral red uptake tests after 24 and 48 h exposure. We observed significant DNA damage at 500 µM of five neonicotinoids in SHSY-5Y cells, while only imidacloprid, thiametoxam, and thiacloprid showed some alterations in HepG2 cells after 24 h exposure using the alkaline comet assay. In conclusion, neonicotinoid insecticides may induce cytotoxicity and DNA damage in cell cultures; therefore, further studies are needed to better understand the toxicity of neonicotinoids.