A Mathematical morphology (MM)-based fault detection technique is presented in this paper to detect fault in rural power distribution systems subjected to resonant grounding. In a resonant grounded power distribution system, the inductance of the ground fault neutralizer creates resonant condition with the zero sequence capacitance of the distribution line which reduces the fault current during line to ground faults. As a result, the fault characteristics change during the faults which make it difficult to detect. The proposed technique is designed to cope with new challenges to detect these types of faults. The main features of the proposed MM-based method are simple, fast, and accurate. The performance of the proposed approach is evaluated on an IEEE standard test system through simulation using MATLAB/SimPowerSystems. Results show that the proposed method can detect any type of faults, distinguish faults from load changes as well as permanent and temporary faults, and identify the faulty phase/s.