It is of great importance to estimate the spatial characteristics of a jointed rock mass in a diverse range of disciplines. Although electrical resistivity prospecting can facilitate this, limited attempts have been made to perform electric field analyses of a jointed rock mass. Considering discontinuity characteristics, we derive closed-form electrical resistivity equations for a jointed rock mass and for a rock mass with a spherical or platy anomaly via electric field analyses. A parametric study shows that the electrical resistivity is affected by the location, size, and characteristics of anomalies, as well as the characteristics of jointed rock masses. Laboratory test results confirmed the validity and accuracy of the derived electrical resistivity equations. Thus, the derived electrical resistivity equations can be effectively utilized to detect an anomaly in a jointed rock mass through a proper inversion of multiple measurements.