Availability of potable water is a problem especially in developing countries as their water sources are often contaminated biologically as well as by heavy metals. Electrochemical methods are suitable for field application to monitor heavy metal contents. Noble metal electrodes with large surface areas are the subject of intensive development as they can yield an improved signal-to-noise ratio, enhanced sensitivity, and lower limits of detection (LOD). Here, we present a nanostructured Au microelectrode array subsequently modified by selective electrodeposition in lithographically defined circles within a partially cross-linked gelatin layer. This method increased its surface area by a factor of ≈1440 in comparison with an original lithography-based prepared array. The Au surface properties can be tailored by a degree of gelatin layer cross-linking. We used this array for an ultrasensitive detection of the As3+ ions content by stripping voltammetry achieving LOD of ≈0.0212 parts per billion (signal-to-noise ratio = 3.3), 470× below the content limit recommended by the World Health Organization for potable water. These nanostructured arrays were used to detect ions of other metals such as Cr, Cd, Hg, Cu, and Sb. In combination with a portable electrochemical device, we can envision an ultrasensitive heavy metal detection system for field application to monitor heavy metal contamination.