Magnetic Co clusters have been electrodeposited from an aqueous electrolyte onto Au surfaces in an electrochemical scanning tunneling microscope (STM). In a two-step electrochemical process, Co is first deposited onto a Au STM tip, then completely dissolved, and locally deposited onto the substrate underneath the STM tip due to local Co^{2 +} oversaturation, which results in a laterally varying increase of the Co/Co^{2 +} Nernst potential at the substrate surface. Mechanical tip-sample contacts or creation of substrate defects can be excluded. The structure size is of the order of the STM tip apex diameter, and is in detail determined by the substrate potential.