OBJECTIVE. The objective of our study was to retrospectively determine the prevalence and image findings of extraarticular talocalcaneal coalition with os sustentaculum, a type of talocalcaneal coalition that does not appear in current classification systems, in patients with an imaging diagnosis of foot coalition. MATERIALS AND METHODS. This study was performed using a database query of radiology reports of ankle and foot CT or MRI examinations performed from August 2001 to November 2013. Eighty-one patients were identified through a keyword search of the database for "talocalcaneal coalition," "tarsal coalition," "coalition," or "os sustentaculum." Imaging features of CT or MRI findings were evaluated. Chart review was used to identify demographic information. RESULTS. Extraarticular talocalcaneal coalition with os sustentaculum was diagnosed in 13 patients (nine men, four women), which represents a prevalence of 16.0% (13/81) in all foot coalitions and 24.1% (13/54) in all talocalcaneal coalitions. Four of 13 patients underwent surgical resection, and histology was obtained in three patients. Nine patients who had no history of trauma were symptomatic and all patients with bone marrow edema at the coalition sites on MRI (n = 5) were also symptomatic. Coexisting extraarticular talocalcaneal coalition with os sustentaculum and intraarticular talocalcaneal coalition were observed in 11 of 13 patients. CONCLUSION. The os sustentaculum is a component of extraarticular talocalcaneal coalitions and as such is usually related to the presence of symptoms. If a patient with an os sustentaculum has symptoms in the medial talocalcaneal joint area, an extraarticular talocalcaneal coalition related to the os sustentaculum should be considered.