The Bosworth fracture dislocation is a rare type of ankle fracture and is usually unrecognized on initial radiographs, therefore early open reduction is recommended. This study reports a 51-year-old female with uncommon imaging and clinical features. Preoperative X-ray, computed tomography, and magnetic resonance imaging of the ankle showed posterior detached dislocation of the intact distal fibula, the loose osteochondral fragments located in the tibiotalar joint, and the rupture of the deltoid ligament. Due to the initial infected wound on the severely swollen ankle, delayed arthroscopy-assisted closed reduction and internal fixation, removal of loose bodies, and repair of the deltoid ligament were performed, 19 days after injury. Postoperative imaging, including the X-ray, computed tomography, and magnetic resonance imaging, demonstrated the anatomic reduction of the ankle joint. After 6 months of follow-up, the patient gained a pain-free motion of the ankle with a range of passive 10° dorsiflexion and 40° plantar flexion, and resumed her normal gait and activity. This report indicates that the detached posterior dislocation of the intact distal fibula is a rare variant of the Bosworth fracture dislocation, and suggests that arthroscopy-assisted closed reduction and removal of loose osteochondral fragments in joint space are useful technique for this special type of the Bosworth lesions.