Os trigonum syndrome is a clinical disorder characterized by posterior ankle pain which occurs in excessive plantar flexion. The pain is elicited by the impingement of os trigonum between the calcaneus and the posterior edge of tibial plafond. Mostly, symptoms can be improved with nonsurgical management, however surgery is required for refractory cases. We report of a case of os trigonum syndrome in a female ballet dancer, which was successfully treated with subtalar arthroscopic excision of os trigonum.