Abstract Fracture of the ulnar sesamoid of the metacarpophalangeal joint of the thumb is rare. Injury may be caused as a result of hyperextension or, less frequently, direct trauma. A sesamoid may be incompletely ossified, making it difficult to diagnose the fracture on radiographs. Early recognition of this entity leads to appropriate management. The case of a 26-year-old man who sustained a hyperextension injury of the thumb is presented. Radiographs demonstrated a fracture of the ulnar sesamoid of the thumb metacarpophalangeal joint. The literature regarding this uncommon injury is discussed.