This study aimed to report the results of a successful periradicular microsurgery on a mandibular first premolar with a radicular groove. A 38-year-old female reported a sinus tract on her buccal gingiva which was radiographically traced to the middle third of the mesial side of the root of her mandibular right first premolar (#44). A radicular groove and an accessory canal in the middle of the radicular groove were found using cone-beam computed tomography. After endodontic treatment, the sinus tract did not disappear; thus, periradicular microsurgery was performed. The radicular groove and the accessory canal were prepared and filled with fastsetting MTA without resectioning the root apex. After 3 weeks, the sinus tract disappeared, and complete healing was revealed on the periapical radiograph at a 5-month follow-up. Therefore, periradicular microsurgery is recommended in mandibular first premolars with radicular grooves on the accessible mesial side of the root.