The common postoperative complications of the extraction of third molars are frequently reported; however, reports about osteomyelitis of the mandible caused by late fracture following third molar extraction are rare. Here, we report a case of osteomyelitis of the mandible caused by late fracture following third molar extraction. A 38-year-old Japanese man was referred to the surgery department with chief complaints of dull pain and swelling in the right masseteric region and paresthesia of his lower lip and mental region in March 2018. A family dentist removed his lower third molar in the right side in January 2018. When the patient was chewing an innards stew 23 days after the procedure, he heard a cracking sound from the right mandible. Thus, we diagnosed the patient as having osteomyelitis of the mandible caused by late fracture following third molar extraction and performed sequestrectomy and curettage under general anesthesia in April 2018. In conclusion, it is necessary to recognize the possibility that late fracture following third molar extraction can cause osteomyelitis. Furthermore, once osteomyelitis of the mandible caused by late fracture occurred, early and appropriate treatment is necessary because the osteomyelitis may progress rapidly and in some cases may result in pathological fracture.