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High-resolution electrochemical deposition of silver nanostructures on insulating atomically flat mica surfaces in humid air was achieved with a scanning tunneling microscope (STM) operating in a scanning electrochemical microscopy mode. The current is faradaic and flows between the tip and a surface contact on the mica substrate through a thin water layer on the mica surface. The thickness and conductivity of the water layer governs the magnitude of the faradaic current. Modified low current STM and conventional tungsten STM tips were used for the silver deposition. The tip was held at a negative bias of -1.2 to -1.4 V (tip vs substrate), with setpoint currents of 1 -1.7 pA. The sizes and shapes of the nanostructures were controlled by the tip scan rates and were restricted to the scan areas under the tip.