In Old Norse, seiðr (sometimes anglicized as seidhr, seidh, seidr, seithr, seith, or seid) was a type of magic practiced in Norse society during the Late Scandinavian Iron Age. The practice of seiðr is believed to be a form of magic which is related to both the telling and the shaping of the future. Connected to the Old Norse religion, its origins are largely unknown, and its practice gradually declined after the Christianization of Scandinavia. Accounts of seiðr later made it into sagas and other literary sources, while further evidence of it has been unearthed by archaeologists. Various scholars have debated the nature of seiðr, some of them have argued that it was shamanic in context, involving visionary journeys by its practitioners.

Seiðr practitioners were of both sexes, with sorceresses being variously known as vǫlur, seiðkonur and vísendakona. There were also accounts of male practitioners, who were known as seiðmenn (or seiðmaðr in the singular). In many cases these magical practitioners would have had assistants to aid them in their rituals.

In pre-Christian Norse mythology, seiðr was associated with both the god Óðinn, a deity who was simultaneously responsible for war, poetry and sorcery, and the goddess Freyja, a member of the Vanir who was believed to have taught the practice to the Æsir.

In the 20th century, adherents of various modern Pagan new religious movements adopted forms of magico-religious practice which include seiðr. The practices of these contemporary seiðr-workers have since been investigated by various academic researchers who are operating in the field of pagan studies.

Seiðr is believed to come from Proto-Germanic *saiðaz, cognate with Lithuanian saitas, 'tie, tether' and Proto-Celtic *soito- 'sorcery' (giving Welsh hud, Breton hud 'magic'), all derived from Proto-Indo-European *soi-to- 'string, rope', ultimately from the Proto-Indo-European root *seH2i- 'to bind'.

Related words in Old High German (see German Saite, used both in string instruments and in bows) and Old English refer to 'cord, string', or 'snare, cord, halter' and there is a line in verse 15 of the skaldic poem Ragnarsdrápa that uses seiðr in that sense. However, it is not clear how this derivation relates to the practice of seiðr. It has been suggested that the use of a cord in attraction may be related to seiðr, where attraction is one element of the practice of seiðr magic described in Norse literature and with witchcraft in Scandinavian folklore. However, if seiðr involved "spinning charms", that would explain the distaff, a tool used in spinning flax or sometimes wool, that appears to be associated with seiðr practice. In any case, the string relates to the "threads of fate", that the Nornir spin, measure, and cut.

Old English terms cognate with seiðr are -siden (as a suffix in the noun ælfsiden, in various inflected forms) and sidsa, both of which are attested only in contexts that suggest that they were used by elves (ælfe); these seem likely to have meant something similar to seiðr. Among the Old English words for practitioners of magic are wicca (m.) or wicce (f.), the etymons of Modern English 'witch'.

In the Viking Age, the practice of seiðr by men had connotations of unmanliness or effeminacy, known as ergi, as its manipulative aspects ran counter to masculine ideal of forthright, open behavior.^{[citation needed]} Freyja and perhaps some of the other goddesses of Norse mythology were seiðr practitioners, Óðinn was accused by Loki in the Lokasenna of being "unmanly" to which Odin replied with: "Knowest thou that I gave to those I ought not – victory to cowards? Thou was eight winters on the earth below, milked cow as a woman, and didst there bear children. Now that, methinks, betokens a base nature."