In mythology, folklore and speculative fiction, shapeshifting is the ability to physically transform oneself through unnatural means. The idea of shapeshifting is found in the oldest forms of totemism and shamanism, as well as the oldest existent literature and epic poems such as the Epic of Gilgamesh and the Iliad. The concept remains a common literary device in modern fantasy, children's literature and popular culture. Examples of shape-shifters include changelings, jinns, kitsunes, vampires, and werewolves, along with deities such as Loki and Vertumnus.

Popular shapeshifting creatures in folklore are werewolves and vampires (mostly of European, Canadian, and Native American/early American origin), ichchhadhari naag (shape-shifting cobra) of India, shapeshifting fox spirits of East Asia such as the huli jing of China, the obake of Japan, the Navajo skin-walkers, and gods, goddesses and demons and demonesses such as the Norse Loki or the Greek Proteus. Shapeshifting to the form of a wolf is specifically known as lycanthropy, and creatures who undergo such change are called lycanthropes. It was also common for deities to transform mortals into animals and plants.

The prefix "were-" comes from the Old English word for "man".

While the popular idea of a shapeshifter is of a human being who turns into something else, there are numerous stories about animals that can transform themselves as well.

Examples of shapeshifting in classical literature include many examples in Ovid's Metamorphoses, Circe's transforming of Odysseus's men to pigs in Homer's The Odyssey, and Apuleius's Lucius becoming a donkey in The Golden Ass. Proteus was known among the gods for his shapeshifting; both Menelaus and Aristaeus captured him to obtain information, and they succeeded only by holding on through his many transformations. Nereus told Heracles where to find the Apples of the Hesperides for the same reason.

The Oceanid Metis, the first wife of Zeus and the mother of the goddess Athena, was believed to be able to change her appearance into anything she wanted. In one story, her pride led Zeus to trick her into transforming into a fly. He then swallowed her because he feared that he and Metis would have a son who would be more powerful than Zeus himself. Metis, however, was already pregnant. She stayed alive inside his head and built an armor for her daughter. The banging of her metalworking made Zeus have a headache, so Hephaestus clove his head with an axe. Athena sprang from her father's head, fully grown, and in battle armor.

In Greek mythology, the transformation is often a punishment from the gods to humans who crossed them.

While the Greek gods could use transformation punitively — such as Medusa, who turned to a monster for having sexual intercourse (raped in Ovid's version) with Poseidon in Athena's temple — even more frequently, the tales using it are of amorous adventure. Zeus repeatedly transformed himself to approach mortals as a means of gaining access: