 TiO2 nanotubes (TiO2 NTs) sensitized with Co-doped CdSe quantum dots (Co-CdSe QDs) were synthesized through an ultrasound-assisted hydrothermal method. Crystallinity and morphology of the prepared samples were characterized by X-ray diffraction and field-emission scanning electron microscopy. Atomic composition was investigated using an energy dispersive spectrometer. Vibration features and phonon modes were confirmed with a Raman spectrometer. Chemical composition and electronic structure were analyzed through X-ray photoelectron spectroscopy. Photoelectrochemical behavior was measured with an electrochemical workstation under 300 W Xe lamp irradiation at room temperature. Results indicated the excellent stability and photoelectrochemical performance of the prepared samples. Compared with TiO2 NTs (0.06 mA cm−2), CdSe QD-sensitized TiO2 NTs (CdSe/TiO2 NTs) exhibited higher photoresponse (0.305 mA cm−2), whereas Co10at.%–CdSe QD-sensitized TiO2 NTs (Co10.at.%–CdSe/TiO2 NTs) showed the highest photoresponse with a stable photocurrent density up to 0.47 mA cm−2.