The aim of the study was to report a clinical case of endodontic treatment of a maxillary first premolar with three root canals using an optical microscope and rotary instrumentation technique. The main complaint of the patient, a 16-year-old girl, was pain in tooth 14. After clinical and radiographic examination, irreversible pulpitis was diagnosed. An alteration in the middle third of the pulp chamber radiographically observed suggested the presence of three root canals. Pulp chamber access and initial catheterization using size number 10 K-files were performed. The optical microscope and radiographic examination were used to confirm the presence of three root canals. PathFiles #13, #16, and #19 were used to perform catheterization and ProTaper files S1 and S2 for cervical preparation. Apical preparation was performed using F1 file in the buccal canals and F2 in the palatal canal up to the working length. The root canals were filled with Endofill sealer by thermal compaction technique using McSpadden #50. The case has been receiving follow-up for 12 months and no painful symptomatology or periapical lesions have been found. The use of technological tools was able to assist the endodontic treatment of teeth with complex internal anatomy, such as three-canal premolars.