Ni(II)-doped TiO₂ nanoparticles were fabricated by a simple and reliable hydrothermal method. Morphology, crystallographic structure and light-absorption properties of the formed photocatalysts were characterized. In addition, both the adsorption phenomena and photodegradation of two types of azo dyes in the presence of Ni(II)-doped TiO₂ were investigated. The experimental results showed that the dye adsorption follows the Langmuir adsorption pathway, and the dye degradation was significantly enhanced by doping of Ni ions into TiO₂ photocatalysts. The optimal doping of Ni ions for the removal of RB5 and 3R dyes was found to be 1%, giving a degradation efficiency 3 times for RB5 dye and 3.5 times for 3R dye higher than those of un-doped TiO₂. Furthermore, the underlying kinetic mechanism was systematically studied, in which the second-order kinetic model was found to be the explicit mechanism for dye degradation on TiO₂ with Ni(II) doping.