In Norse mythology, a fylgja (Old Norse: [ˈfylɡjɑ], plural fylgjur [ˈfylɡjuz̠]) is a supernatural being or spirit which accompanies a person in connection to their fate or fortune. They can appear to people in their sleep as dream-women, or appear to them while awake, often in the disembodied spiritual form of an enemy.

The word fylgja means "to accompany". The term fylgja is typically translated into English as "fetch", a similar being from Irish folklore.

The term fylgja also has the meaning of "afterbirth, caul", and it has been argued by Gabriel Turville-Petre (cf. § Placenta origins) that the concept of the supernatural fylgja cannot be completely dissociated from this secondary meaning; in fact, there may well be a connection to the hamr, referring to the skin used by shapeshifters (hamramr, cf. fjaðrhamr).

The fylgja is a ghost who associates with (or, for lack of a better word, stalks or shadows) a particular individual, and may be characterized as a "guardian spirit". However, contrary to its name meaning 'follower', it generally moves ahead of its host, making a kind of "contact" with the person before they arrive at some key spot. The fylgja will, however, follow a person when they are near death.

A fylgja is sometimes associated with a particular family or clan, in which case it is called an ættarfylgja (pl. ættarfylgjur 'family followers'). A closely related type of fylgja is the "dream woman", as appears in Gísla saga (Cf. § Sleep and dreams below). It is contended that the Icelandic mar or mara (the folkloric "[night]mare") is a dream fylgja which has strayed from its assigned host and visiting the dreams of others, which tend to be more sinister than when visiting its usual host or ward.

The fylgja is said to take on either an animal form or a female human form, and this is due to a conflation of two distinct types of spirits, according to Else Mundal; the term fylgja, she argues, was first associated with the animal spirit, then later applied to the woman-spirit type.

The Icelandic word fylgja can also mean "placenta" or "afterbirth of a child" and the folkloric supernatural connection made between child and afterbirth may be the origins of the fylgja as a concept. According to some, the fylgja takes on the form of whatever animal that first showed itself and consumed the newborn baby's afterbirths; these creatures taking on such forms as mice, sheep, dogs, foxes, cats, and raptors, birds of prey, or carrion eaters.

Thus, while the fylgja usually was a female ghost in the shape of a woman, it sometimes took on the shape of animals (also female animals according to Mundal). The animal fylgjur typically came in the form of a dog, but also as various other land or even sea creatures.