The accessory soleus muscle is a rare anatomical variant which, although congenital in origin, may manifest in the second and third decades of life as a painful ankle mass or an asymptomatic ankle soft tissue swelling. We report a symptomatic accessory soleus muscle in a 21-year-old male soldier that was diagnosed and followed-up on MRI. Initial MRI showed a mass with signal characteristics of normal muscle, but in an abnormal location. There was increased intrafascial fluid and perimuscular oedema around the accessory soleus muscle. Following conservative treatment, repeat MRI showed resolution of this intrafascial fluid collection and perimuscular oedema, concurrent with relief of the patient's painful symptoms.