In this study, the impact of carbon quantum dots (CQDs) on the photocatalytic performance of TiO₂ in aqueous medium was investigated under UV and visible light. CQDs were synthesized through a solvothermal procedure, and the effect of synthesis conditions on their optical properties were optimized. The CQDs/TiO₂ nanocomposites were prepared via a hydrothermal method using CQDs with the extreme band-gaps (i.e. CQDs450 and CQDs550). The physicochemical properties of CQDs and CQDs/TiO₂ samples were studied by various characterization techniques. According to the High-Resolution Tunneling Electron microscopy (HRTEM) micrographs, the synthesized spherical CQDs had diameters smaller than 5 nm. The Diffuse Reflectance Spectroscopy (DRS) analysis revealed that incorporation of CQDs into nanocomposite samples causes a remarkable shift in absorption edge to the visible region, in comparison with pure TiO₂. According to the experimental data, the nanocomposite sample containing CQDs with the higher absorption edge, i.e. CQDs550/TiO₂, showed a superior performance in photocatalytic removal of methylene blue (MB) under visible light. In this regard, the removal percent of MB on CQDs550/TiO₂ and CQDs450/TiO₂ nanocomposites, with 4%wt CQDs, under visible light (λ > 420 nm) were 92.1 % and 53.4 % after 120 min of irradiation at pH = 11 and 150 mgcat./L catalyst dosage, which are respectively 2.4 and 1.4 times higher than that on pure TiO₂. However, under UV irradiation the observed photocatalytic performances were in the reverse order. Finally, two different reaction mechanisms have been proposed to explain the obtained results on CQDs/TiO₂ samples, based on possible electron transfer between TiO₂ and CQDs nanoparticles under UV and Visible lights.