The objective of this case study is to report the endodontic treatment of a mandibular premolar with a C-shaped root canal configuration based on the use of cone-beam computed tomography (CBCT) as a diagnostic aid and in treatment planning. A 14-year-old patient was referred for endodontic treatment of tooth No. 21. Upon examination of the CBCT, C-shaped canal anatomy was verified. The chemical-mechanical preparation was carried out with a rotary file system (rotary files 25.08, 30.05, and 35.05) and the use of 2.5% sodium hypochlorite as an auxiliary chemical substance. Clinical and radiographic follow-up was performed 6 months and 1 year after endodontic treatment. A periapical radiolucency revealed a progressive reduction, suggesting the evolution of tissue repair. This case study confirms that to achieve a favorable prognosis of endodontic treatment, in-depth knowledge of the internal anatomy of the root canal system, along with its variations, is necessary. In this regard, CBCT is an important tool to assist in obtaining the correct diagnosis and understanding the anatomical complexity to be treated. Variations of C-shaped canals are challenging due to difficulties encountered during the stages of chemical-mechanical preparation and filling. Thus, means of enhancing disinfection are needed.