The presence of an os sustentaculum bone is extremely rare. Given the scarcity of clinical literature reporting such a finding, the condition may be misdiagnosed as a fracture of the sustentaculum tali. We describe such an incident in a 16-year-old male athlete presenting for a recent ankle sprain with no other history of trauma or pain on the medial aspect of the ankle. The original computed tomographic scan reported a possible nonunion of a sustentaculum fracture, or peripherally corticated ossification in association with a possible talocalcaneal fibrocartilaginous coalition. Given no history of high-level trauma to the area, the clinical presentation, and the radiographic findings, the more likely diagnosis was confirmed to be an os sustentaculum. The goal of the authors of this article is to report such findings to raise awareness of a rare clinical presentation to avoid misdiagnosis.