Variations in leg muscle are uncommon. Literature on this subject is scarce, but when those variations are reported they may cause alterations in joint mechanics or cause some discomfort in the leg and foot. The accessory soleus muscle (ASM) is considered an unusual anatomical variation, with an  incidence of 0.5-6.0% in the population through studies in cadavers. During routine preparation of study material in the dissection room of the anatomy laboratory of the Escola Superior de Ciências da Santa Casa de Misericórdia de Vitória/ES – Brazil, an ASM was found in the right inferior limb of a male cadaver fixed in 10% formalin. This supernumerary muscle was 3 cm wide, 9 cm long and 1 cm thick in its most voluminous part, in typical penniform fibers arrangement. It was located in the posteromedial region of the ankle, anterior to the Achilles tendon and posterior to the deep muscles of the leg compartment. Its anterior face covered the tibial nerve and the posterior tibial vessels, while its lower half was covered by the flexor retinaculum into the tarsal tunnel. Reports in the literature show possible compression of a neurovascular bundle because of its intimal position within the tarsal tunnel, which could result in ischemic compartment syndrome.