Systematic control of electrical properties is a challenging task in polycrystalline materials having significant electrical heterogeneity. In particular, as most aliovalent dopants in oxides usually play a single role of either a donor or an acceptor, controlling the respective electronic conductance of grains and grain boundaries in a separate manner by simple doping is not easily achieved. Using the polycrystalline perovskite Ca1/4Cu3/4TiO3, the grains of which have relatively high conductance and the grain boundaries reveal much lower conductance, we demonstrate the donoreacceptor bifunctionality of dysprosium (Dy) and its impact on the variation of electrostatic potential barriers at grain boundaries. DC currentevoltage measurements and impedance spectroscopy analyses consistently show that Dy selectively affects the grain-boundary conductance without a substantial change of conductance in the grains. Atomic-scale scanning transmission electron microscopy also visualizes the distinct site occupancy of Dy at the Ti columns in the grain-boundary region in contrast to the Dy occupation at the Ca columns in the bulk grains, directly showing that Dy is a donor in the bulk and simultaneously an acceptor in the grainboundary region. In addition to providing an efficient way of tuning the electrostatic potential barriers, this bifunctional effect of Dy emphasizes the significance of understanding the correlation between the atomic-level site occupancy of dopants and the overall electrical properties.