Spatially separated electron donors and acceptors in sediment can be exploited by the so-called “cable bacteria.” Electric potential microelectrodes (EPMs) were constructed to measure the electric fields that should appear when cable bacteria conduct electrons over centimeter distances. The EPMs were needle-shaped, shielded Ag/AgCl half-cells that were rendered insensitive to redox-active species in the environment. Tip diameters of 40 to 100 µm and signal resolution of approximately 10 μV were achieved. A test in marine sediments with active cable bacteria showed an electric potential increase by approximately 2 mV from the sediment-water interface to a depth of approximately 20 mm, in accordance with the location and direction of the electric currents estimated from oxygen, pH, and H2S microprofiles. The EPM also captured emergence and decay of electric diffusion potentials in the upper millimeters of artificial sediment in response to changes in ion concentrations in the overlying water. The results suggest that the EPM can be used to track electric current sources and sinks with submillimeter resolution in microbial, biogeochemical, and geophysical studies.