Localized electrochemical deposition of small Co clusters on Au (111) has been achieved by using the tip of a scanning tunneling microscope (STM) as an electrochemical nanoelectrode. In contrast to most of the reported techniques to create nanostructures, this approach avoids irreversible modifications of both substrate and deposit during the preparation process. A special polarization routine of the STM tip allows a purely electrochemical growth of Co clusters with diameters as small as 15 nm.