Aim: This case report aims to present the endodontic treatment applied to the maxillary left central tooth with advanced external root resorption and previous root canal treatment. Case: A 15-year-old systemically healthy female patient presented to our clinic with a radiolucent lesion detected on the periapical radiograph of the maxillary left central tooth, along with a history of previous root canal treatment and advanced external root resorption. Cone-beam computed tomography evaluation confirmed the diagnosis of external root resorption. Clinical examination revealed a positive response to percussion and palpation tests and a sinus tract was observed inside the mouth, apically to the tooth. According to the Miller mobility classification, it was determined to have class 2 mobility. Treatment planning was carried out, and the patient was informed accordingly. In the first session, root canal filling was removed using hand instruments with a 2% taper under rubber dam isolation, and mechanical preparation of the canal was completed with an ISO 80 K-file. Calcium hydroxide paste was placed in the canal, and a follow-up appointment was scheduled for 14 days later. Calcium hydroxide medication was repeated twice. In the final session, 2% NaOCl and 17% EDTA were used as irrigation solutionsfor the patient whose sinus tract had healed and symptoms had subsided. The resorption area at the apex was filled with Mineral Trioxide Aggregate (MTA), and the remaining portion was filled with thermoplastic gutta-percha. Results: Based on the 3 and 6-month radiographic and clinical follow-ups, the tooth was found to be asymptomatic. The patient’s follow-up is ongoing. Conclusion: This case presentation demonstrates that teeth with advanced external root resorption can be successfully treated using MTA and thermoplastic root canal filling techniques.