The púca (Irish for spirit/ghost; plural púcaí), puca (Old English for goblin), also pwca, pookah, phouka, and puck, is a creature of Celtic, English, and Channel Islands folklore. Considered to be bringers both of good and bad fortune, they could help or hinder rural and marine communities. Púcaí can have dark or white fur or hair. The creatures were said to be shape-changers that could take the appearance of horses, goats, cats, dogs, and hares. They may also take a human disguise, which includes various animal features, such as animal ears or a tail.

The origin of the name is debated, with some theorising that it originated in the Irish language, but with a different spelling as there was no p sound in Primitive Irish. It appears, from place name evidence, to have been in use as early as the 8th century. Since it is a 'cultural' rather than a practical word that might be used in trading, it is thought to reflect greater cultural contact between Celtic and Germanic cultures in the early medieval period than had been thought.

The púca has counterparts throughout the Celtic and Germanic cultures of Northwest Europe. For instance, in Scandinavian languages, we find, according to the OED, "Old Icelandic púki mischievous demon, the Devil, Faroese púki, Norwegian (originally and chiefly regional) puke devil, evil spirit, mischievous person,^{[citation needed]} Old Swedish puke devil, evil spirit, Swedish (now chiefly regional) puke evil spirit, devil, goblin), Old Danish puge evil spirit". In Welsh mythology, it is named the pwca and in Cornish the Bucca (thus being related in etymology and milieu to the bugaboo). In the Channel Islands, the pouque were said to be fairies who lived near ancient stones; in Norman French of the Islands (e.g. Jèrriais), a cromlech, or prehistoric tomb, is referred to as a pouquelée or pouquelay(e); poulpiquet and polpegan are corresponding terms in Brittany.

The púca may be regarded as being either menacing or beneficial. Fairy mythologist Thomas Keightley said "notions respecting it are very vague", and in a brief description gives an account collected by Croker from a boy living near Killarney that "old people used to say that the Pookas were very numerous ... long ago ... , were wicked-minded, black-looking, bad things ... that would come in the form of wild colts, with chains hanging about them", and that did much to harm unwary travellers. Also, little bad boys and girls were warned not to eat overripe blackberries, because this was a sign that the pooka has entered them.

One theme of the púca's folklore is their proclivity for mischief. They are commonly said to entice humans to take a ride on their back, giving the rider a wild and terrifying journey before dropping the unlucky person back at the place they were taken from. This lore bears similarities to other Irish folk creatures, such as the daoine maithe (good people) or the slua sí (fairy host), said to target humans on the road or along their regular "passes". These human encounters of the púca tend to occur in rural, isolated places, far from settlements or homes.

While púca stories can be found across northern Europe, Irish tales specify a protective measure for encountering a púca. It is said that the rider may be able to take control of the púca by wearing sharp spurs, using those to prevent being taken or to steer the creature if already on its back.

A translation of an Irish púca story, "An Buachaill Bó agus an Púca", told by storyteller Seán Ó Cróinín, describes this method of control of the púca as done by a young boy who had been the creature's target once before:

... the farmer asked the lad what had kept him out so late. The lad told him.