The word werewolf comes from the Old English word werwulf, a compound of wer "man" and wulf "wolf". The only Old High German testimony is in the form of a given name, Weriuuolf, although an early Middle High German werwolf is found in Burchard of Worms and Berthold of Regensburg. The word or concept does not occur in medieval German poetry or fiction, gaining popularity only from the 15th century. Middle Latin gerulphus Anglo-Norman garwalf, Old Frankish *wariwulf. Old Norse had the cognate varúlfur, but because of the high importance of werewolves in Norse mythology, there were alternative terms such as ulfhéðinn ("one in wolf-skin", referring still to the totemistic or cultic adoption of wolf-nature rather than the superstitious belief in actual shapeshifting). In modern Scandinavian also used was kveldulf "evening-wolf", presumably after the name of Kveldulf Bjalfason, a historical berserker of the 9th century who figures in the Icelandic sagas.

The term lycanthropy, referring both to the ability to transform oneself into a wolf and to the act of so doing, comes from Ancient Greek λυκάνθρωπος lukánthropos (from λύκος lúkos "wolf" and ἄνθρωπος, ánthrōpos "human"). The word does occur in ancient Greek sources, but only in Late Antiquity, only rarely, and only in the context of clinical lycanthropy described by Galen, where the patient had the ravenous appetite and other qualities of a wolf; the Greek word attains some currency only in Byzantine Greek, featuring in the 10th-century encyclopedia Suda. Use of the Greek-derived lycanthropy in English occurs in learned writing beginning in the later 16th century (first recorded 1584 in The Discoverie of Witchcraft by Reginald Scot, who argued against the reality of werewolves; "Lycanthropia is a disease, and not a transformation." v. i. 92), at first explicitly for clinical lycanthropy, i.e. the type of insanity where the patient imagines to have transformed into a wolf, and not in reference to supposedly real shapeshifting. Use of lycanthropy for supposed shapeshifting is much later, introduced ca. 1830.

Slavic uses the term vlko-dlak (Polish wilkołak, Czech vlkodlak, Slovak vlkolak, Serbo-Croatian вукодлак - vukodlak, Slovenian volkodlak, Bulgarian върколак/vrkolak, Belarusian ваўкалак/vaukalak, Ukrainian вовкулака/vovkulaka), literally "wolf-skin", paralleling the Old Norse ulfhéðinn. However, the word is not attested in the medieval period. The Slavic term was loaned into modern Greek as Vrykolakas. Baltic has related terms, Lithuanian vilkolakis and vilkatas, Latvian vilkatis and vilkacis. The name vurdalak (вурдалак) for the Slavic vampire ("ghoul, revenant") is a corruption due to Alexander Pushkin, which was later widely spread by A.K. Tolstoy in his novella The Family of the Vourdalak (composed in French, but first published in a Russian translation in 1884).