Titanium dioxide (TiO₂) nanomaterials have garnered extensive scientific interest since 1972 and have been widely used in many areas, such as sustainable energy generation and the removal of environmental pollutants. Although TiO₂ possesses the desired performance in utilizing ultraviolet light, its overall solar activity is still very limited because of a wide bandgap (3.0–3.2 eV) that cannot make use of visible light or light of longer wavelength. This phenomenon is a deficiency for TiO₂ with respect to its potential application in visible light photocatalysis and photoelectrochemical devices, as well as photovoltaics and sensors. The high overpotential, sluggish migration, and rapid recombination of photogenerated electron/hole pairs are crucial factors that restrict further application of TiO₂. Recently, a broad range of research efforts has been devoted to enhancing the optical and electrical properties of TiO₂, resulting in improved photocatalytic activity. This review mainly outlines state-of-the-art modification strategies in optimizing the photocatalytic performance of TiO₂, including the introduction of intrinsic defects and foreign species into the TiO₂ lattice, morphology and crystal facet control, and the development of unique mesocrystal structures. The band structures, electronic properties, and chemical features of the modified TiO₂ nanomaterials are clarified in detail along with details regarding their photocatalytic performance and various applications.