This study was aimed at assessing the performance of electrical resistivity tomography (ERT) applied to the investigation of seepage in earth dams through a case study. Several abnormal leaks appeared on the downstream face of an earth dam after the dam was reconstructed to raise the maximum reservoir water level. A study was conducted to investigate the mechanism of the abnormal leakage with the assistance of ERT. Three two-dimensional (2D) ERT survey lines were deployed on the left abutments, dam crest, and downstream shell, respectively. Periodic measurements were additionally collected on the downstream shell for time-lapse measurements. To gain confidence and avoid over interpretation, the results of 2D ERT were appraised by forward modeling and synthetic inversion. Combining ERT results with geotechnical monitoring data clearly indicates the likely mechanism of abnormal seepage. Time-lapse measurements further support the inducted mechanism. Integration of ERT exploration with time-lapse ERT measurements and geotechnical monitoring data was demonstrated to better understand the possible mechanism of the abnormal seepage.