Local softening was observed for the nanoindentation of a polycrystalline ZnS in an electric field. The indentation load was in the range of 35–100 μN, and the electric field intensity was in the range of 0.5–2 V/mm. The indentation hardness decreased with increasing the electric field intensity. This effect was likely due to the electromechanical interaction which changed the effective resolved shear stress and the average velocity of charged dislocations. A critical increment in the resolved shear stress was obtained at which the average velocity of the charged dislocations is the same as that in the absence of electric field.