Legio V Parthica (the "Fifth Parthian Legion") was a legion of the Roman Empire (and later the Byzantine Empire) garrisoned in Amida, Mesopotamia, established by the Roman emperor Diocletian (284–305), who reorganized the eastern frontier. The legion is described by the historian Ammianus Marcellinus. The cognomen "Parthica" was an archaism, as the Parthian Empire was already replaced by the Sasanian Empire at the time of the establishment of the legion.