BACKGROUND Type Ⅲb dens invaginatus (DI) with a lateral canal located at the mid-third of the root is rarely reported. Here, we report a rare case of type Ⅲb DI in the left upper anterior tooth with a lateral canal that led to persistent periodontitis. CASE SUMMARY A 15-year-old female patient presented with a chief complaint of pain associated with recurrent labial swelling in the area of the left anterior tooth. A diagnosis of type Ⅲb DI and chronic periodontitis was made. Intentional replantation was performed after conventional endodontic treatment failed. After 6 mo, the patient was asymptomatic, but a sinus tract was observed. Cone-beam computed tomography images showed bone loss in the mesial of the mid-root. Based on methylene blue staining and microscopy images, the lateral foramen located at the middle third of the root was surgically treated. After 3 years of follow-up, the clinical findings and radiographic assessment presented a favorable prognosis of bone healing without root absorption or ankylosis. CONCLUSION Type Ⅲb DI with a lateral canal can be successfully treated by root canal treatment, intentional replantation, and surgical therapy.