Highly crystalline ZnO nanostructures were fabricated successfully using a new electrochemical approach based on underpotential deposition (UPD) and simultaneous oxidation of Zn atomic layers from an oxygenated aqueous suspension of ZnO. Electrochemical UPD and the oxidation mechanism of the Zn atomic layers to ZnO were studied by cyclic voltammetry and potential-controlled electrochemical deposition techniques. Scanning electron microscopy and X-ray diffraction (XRD) measurements showed that the electrodeposited ZnO nanostructures are hexagonal in shape and prefer the (0 0 2) orientation. From the XRD patterns and the energy dispersive spectroscopy studies, the purity and crystallinity of the ZnO nanostructures were confirmed. The photoluminescence spectra showed an emission band in the ultraviolet region and another in the blue-green visible region. The photocurrent transients exhibited fast and uniform photocurrent responses, indicating that charge transport in these materials proceeds rapidly and may be associated with the single crystalline structure of ZnO. The present study reports a method of electrodepositing ZnO and other metal oxide nanostructures with controllable morphology and size.