Accessory soleus muscle is an uncommon anatomical variant that may present as a soft-tissue mass in the posteromedial region of the ankle. It is congenital in origin but usually presents in the second or third decade of life. Although it is a rare entity, accessory soleus muscle should be included in the differential diagnosis of soft-tissue swelling of the ankle. Awareness of the clinical presentation and specific findings of computed tomography, magnetic resonance imaging, and electromyography help with diagnosis without surgical exploration. We describe a 30-year-old patient with accessory soleus muscle. Magnetic resonance imaging features of the case are described, and the literature is briefly reviewed. (J Am Podiatr Med Assoc 94(6): 587–589, 2004)