Titanium dioxide nanoparticles (TiO2 NPs) are one of the most widely used nanomaterials in the consumer products, agriculture, and energy sectors. Their large demand and widespread applications will inevitably cause damage to organisms and ecosystems. A better understanding of TiO2 NP toxicity in living organisms may promote risk assessment and safe use practices of these nanomaterials. This review summarizes the toxic effects of TiO2 NPs on multiple taxa of microorganisms, algae, plants, invertebrates, and vertebrates. The mechanism of TiO2 NP toxicity to organisms can be outlined in three aspects: The Reactive Oxygen Species (ROS) produced by TiO2 NPs following the induction of electron–hole pairs; cell wall damage and lipid peroxidation of the cell membrane caused by NP-cell attachment by electrostatic force owing to the large surface area of TiO2 NPs; and TiO2 NP attachment to intracellular organelles and biological macromolecules following damage to the cell membranes. Display omitted