Chronic osteomyelitis with proliferative periostitis, known as Garre’s osteomyelitis, is a type of osteomyelitis characterized by a distinctive gross thickening of the periosteum of bones. Peripheral reactive bone formation can be caused by mild irritation or infection. Garre’s osteomyelitis is usually diagnosed in children and young adults, and the mandible is more affected than the maxilla. The following is a case report of a 12-year-old female patient with Garre’s osteomyelitis of the mandible due to an infection of a root canal-treated tooth. Without surgical intervention, the patient’s symptoms were relieved through nonsurgical root canal re-treatment with long-term calcium hydroxide placement. A cone-beam computed tomography image obtained 6 months after treatment completion displayed complete healing of the periapical lesion and resolution of the peripheral reactive buccal bone. Due to the clinical features of Garre's osteomyelitis, which is characterized by thickening of the periosteum, it can be mistaken for other diseases such as fibrous dysplasia. It is important to correctly diagnose Garre's osteomyelitis based on its distinctive clinical features to avoid unnecessary surgical intervention, and it can lead to minimally invasive treatment options.