The accessory soleus muscle is a rare anatomical variant that usually occurs in adolescents or young adults. And it may manifest as an exertional ankle pain and swelling or as an asymptomatic posteromedial mass. However sometimes it may be mistaken as a soft tissue tumor at the ankle. The purpose of this study is to heighten awareness about the existence of accessory soleus muscle in a swelling at the posteromedial aspect of the ankle and to reduce the failure to recognize this muscle in MRI.