Self-doping by Ti³⁺ and introducing oxygen vacancies in TiO₂ is an important and effective strategy to extend its optical absorption from the UV into the visible region. In this paper, we report the synthesis of a series of Ti³⁺ self-doped TiO₂ nanocrystals via a hydrothermal method using TiCl₃ and (NH₄)₂TiF₆ as the source of Ti³⁺ and Ti⁴⁺, respectively. The oxidation of Ti³⁺ can be inhibited by (NH₄)₂TiF₆ based on Le Chatelier's principle. The samples are characterized by X-ray diffraction, UV-Vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance, field-emission scanning electron microscopy and transmission electron microcopy, respectively. The results indicate that Ti³⁺ is localized in the bulk of the as-prepared TiO₂ nanocrystals, rather than on the surface. In comparison with pristine TiO₂, the Ti³⁺ self-doped TiO₂ nanocrystals exhibit an enhanced photocatalytic activity of 86.3% towards the photodegradation of methylene blue solution under simulated solar light irradiation and 99% under natural solar irradiation. It is found that the molar ratio of Ti⁴⁺/Ti³⁺ used during the synthesis has significant effects on the photocatalytic activity of Ti³⁺ self-doped TiO₂ photocatalysts, since the electronic structures of the resulting Ti³⁺ self-doped TiO₂ nanocrystals can be finely tuned by changing the ratios. The significantly enhanced photocatalytic activity should be due to the self-doping of Ti³⁺ in TiO₂ nanocrystals, which not only increases the optical absorption in the visible light region, but also helps to spatially separate the photogenerated charge carriers.