Background <p>A simple bone cyst (SBC) is a non-neoplastic intraosseous cavity without an epithelial lining. It is usually asymptomatic and has little effect on the surrounding structures. SBC associated with root resorption of the involved tooth has seldom been reported.</p> Case presentation <p>A 63-year-old Korean woman was referred to our hospital for a cystic lesion of the left mandibular second molar. Radiographic examination revealed a well-demarcated radiolucent lesion with severe root resorption in the involved tooth. The tooth was extracted, and surgical exploration revealed an empty cavity with only a small amount of fibrous connective tissue. Histological examination revealed external root resorption in the left mandibular second molar and a cystic structure consisting of fibrous collagenous tissue devoid of an epithelial lining.</p> Conclusions <p>This case of SBC associated with root resorption offers valuable insight into the diagnostic process for radiolucent lesions accompanied by root resorption.</p>