The os trigonum is an accessory bone which is usually asymptomatic and located behind the lateral bulge of the talus. During tough plantar flexion of the ankle, compression of the os trigonum between the posterior malleolus of the tibia and the tuber calcaneus may cause the clinical syndrome named “Os trigonum syndrome”. Os trigonum syndrome is a rare syndrome characterized by posterior foot pain. Diagnosis is based on history, clinical examination and radiological findings. In this study, we presented a case of Os trigonum syndrome who presented with pain and swelling in the posterior left foot and treated conservatively.