Internal root resorption (IRR) is an uncommon but potentially destructive pathological condition characterized by progressive loss of dentin from within the pulp space as a result of odontoclastic activity. It is frequently asymptomatic and often detected incidentally during radiographic examination, underscoring the importance of early diagnosis and timely intervention to prevent structural compromise and tooth loss. This case report describes the successful nonsurgical management of a non-perforating internal root resorption lesion in a mandibular first molar. A 32-year-old male patient presented with pain in the lower left posterior region, and radiographic evaluation revealed a well-defined radiolucent defect in the cervical third of the distal root, consistent with internal root resorption, along with associated periapical pathology. Nonsurgical endodontic therapy was performed, involving thorough chemomechanical debridement, intracanal medication with calcium hydroxide, and obturation using gutta-percha with a bioceramic sealer. The resorptive defect was sealed with mineral trioxide aggregate (MTA) to achieve an effective three-dimensional seal and promote periradicular healing. Clinical and radiographic follow-up at six and twelve months demonstrated resolution of symptoms, healing of periapical lesions, and arrest of the resorptive process. This case highlights the significance of early diagnosis, appropriate case selection, and the use of bioactive materials such as MTA in achieving favorable outcomes in the nonsurgical management of internal root resorption.