Objective To develop a radiological classification system for talocalcaneal coalition suitable for adults. Methods and materials A retrospective review was performed on patients diagnosed with talocalcaneal coalition from July 2001 to November 2011. Based on the cartilaginous or bony nature, facet joint orientation and bony structure morphology, we classified talocalcaneal coalitions into four types: I (linear with or without posterior hooking), II (talar overgrowth), III (calcaneal overgrowth) and IV (complete osseous). Results Seventy feet (59 patients) with talocalcaneal coalition were evaluated by CT (61/70 feet) using multi-planar reformation and/or magnetic resonance imaging (43/70 feet). Type I, II, III and IV coalitions were detected in 45 (64 %), 10 (14 %), 13 (19 %), and 2 feet (3 %), respectively. Fracture fragments were observed in 16 feet (seven Type I and nine Type III coalitions) with hooked or overgrown calcanei and in one foot in the talus (Type I). Eleven patients had bilateral talocalcaneal coalitions; ten patients had coalitions of the same type and one had both Type I and Type III coalitions. Among 48 patients with unilateral involvement, the left and right feet were affected in 26 and 22 patients, respectively. Conclusions A classification system for talocalcaneal coalition based on multi-planar imaging studies was developed. Key Points • A classification system for talocalcaneal coalition based on multi-planar imaging was developed. • The relative frequencies of different talocalcaneal coalition types were determined. • Fracture fragments were easily distinguished and frequently originated from the calcaneus. • Fracture fragments were mostly associated with Type I (linear) with posterior hooking and Type III (calcaneal overgrowth).