Introduction and importance: An adenomatoid odontogenic tumor is a rare medical condition. Large tumor (or several) often appears in the maxillae. In a minority of cases, the tumor(s) appear in the mandible. Case presentation: We report on a case of a 24-year-old female diagnosed with a mandibular adenomatoid odontogenic tumor, a giant tumor measuring approximately 22 x 25 x 17 cm. The tumor was located on the side of the mandible, causing facial deformity, malnutrition, and hemorrhaging. We assessed the patient's overall condition, carried out a resection of the tumor and mandible from the right condyle to the left mandibular angle, and reconstructed the mandibular defect with a fibula free flap. After the treatment, the patient was followed up for 1 year, with no recurrence detected over this period. Clinical discussion: Because adenomatoid odontogenic tumors are benign odontogenic lesions, which are painless and slow-growing, most are surgically removed or treated conservatively. However, the above treatment measures cannot be applied in the case of a giant tumor that causes facial deformity, destroys the entire jawbone, and has complications such as hemorrhaging and malnutrition. After the tumor resection, the defect is still significant. Accordingly, reconstruction using a microsurgical bone flap is an effective method instead. Conclusion: Large adenomatoid odontogenic tumors in the mandible are rare, and treatment cannot follow conventional methods. Accordingly, defect reconstruction after tumor resection is essential.