The most common reason for a retained primary tooth is a congenitally missing permanent successor. Maintaining a grossly decayed retained primary molar is a challenge for pedodontists. When such a retained tooth presents with carious pulp exposure, endodontic treatment can help maintain the space and prevent arch length discrepancy. The purpose of this case report is to emphasize the root canal treatment of a retained mandibular primary molar using Gutta-percha.