A novel nanocomposite of carbon quantum dots (CQDs) and TiO₂ nanotubes was fabricated and its enhanced photocatalytic and photoelectrochemical properties were demonstrated. Carbon quantum dots were obtained by electrochemical-etching graphite electrodes and TiO₂ nanotubes arrays were prepared by anodization methods. Subsequently, CQDs were assembled on the surface of vertically aligned TiO₂ nanotube arrays (CQDs/TiO₂ nanotubes) and the as-prepared samples were characterized by field-emission scanning electron microscopy, X-ray diffraction, UV-vis diffuse reflectance spectroscopy, photoelectrochemical and photocatalytic measurements. XPS measurement shows the presence of carbon species which come from CQDs. A red shift of light absorption edge and more absorption in the visible light region were observed for the resulting samples from the UV-vis diffuse reflectance spectra. An enhanced photocurrent and photopotential were demonstrated for the CQDs sensitized TiO₂ nanotubes under visible light irradiation and the photocurrent density was 2.7 times larger than that of pristine TiO₂ nanotubes. A solar cell was fabricated for further verifying the sensitization of CQDs over TiO₂ nanotubes. Moreover, the photocatalytic activity of CQDs/TiO₂ nanotubes towards the degradation of methylene blue was demonstrated and about 14% enhanced degradation efficiency was obtained with the presence of CQDs. This work developed a simple method to fabricate CQDs and demonstrated the introduction of CQDs to be a new approach for improving the utilization of visible light for TiO₂ nanotube arrays.