The most frequent cause of posterior ankle impingement syndrome involves the os trigonum and the talar process, being called os trigonum syndrome to the symptomatic soft tissue and bone compression at the tibiocalcaneal interval. It is a controversial entity, currently with limited evidence at scientific literature. Os trigonum syndrome is frequently misdiagnosed or tardily diagnosed. A high suspicion level is needed for its early diagnosis, which is essential not only to quickly begin the proper treatment for symptoms relief, but also for prognostic terms.