Calcaneus secundarius is an accessory ossicle of the anterior calcaneal facet identified in up to 5% of the population. Calcaneus secundarius rarely leads to symptoms, but its presence may generate pain around the ankle. The purpose of this case report is to describe this atypical accessory ossicle, which can mimic a calcaneonavicular coalition, and to explain the clinical approach to reach a diagnosis and determine treatment. The authors report the case of a 13-year-old girl who sustained several lateral sprains of the right ankle and had local chronic pain. Clinical findings were consistent with calcaneonavicular coalition but radiographic examinations revealed a calcaneus secundarius. The location of this ossicle may limit the range of motion of the subtalar joint, mimicking a calcaneonavicular coalition. Surgical excision of this bulky accessory ossicle was performed, and this treatment fully resolved the pain and improved subtalar motion.