BACKGROUND: That the peroneus tertius muscle (PT) is a separate entity has been long debated. PT has been variously reported to be part of the extensor digitorum longus muscle, part of the extensor digitorum brevis, or a separate muscle. While pigs have a PT as well as primates, there are no reports of its association with the extensor digitorum longus muscle or extensor digitorum brevis. MATERIALS AND METHODS: In this study, we used gross dissection and Sihler's staining to determine the origin, course, insertion, and innervation of the pig PT. RESULTS: The PT and extensor digitorum longus muscles jointly originated from the femur and ran between the tibialis cranialis and peroneus longus muscles. The PT was inserted at the retinaculum of the metatarsal extensors, tarsal bone, and second metatarsal bone. The branches of the common fibular nerve to the extensor digitorum longus muscle were distributed to the PT. CONCLUSIONS: The innervations suggest that the PT and extensor digitorum longus muscles of the pig were derived from the same muscle mass during development, but were named separately due to differences in their morphology. Furthermore, morphological features suggest that pig PT and human PT are probably different muscles.