In Norse mythology, the einherjar (singular einheri; literally "army of one", "those who fight alone") are those who have died in battle and are brought to Valhalla by valkyries. In Valhalla, the einherjar eat their fill of the nightly resurrecting beast Sæhrímnir, and valkyries bring them mead from the udder of the goat Heiðrún. The einherjar prepare daily for the events of Ragnarök, when they will advance for an immense battle at the field of Vígríðr.

The einherjar are attested in the Poetic Edda, compiled in the 13th century from earlier traditional sources, the Prose Edda, written in the 13th century by Snorri Sturluson, the poem Hákonarmál (by the 10th century skald Eyvindr skáldaspillir) as collected in Heimskringla, and a stanza of an anonymous 10th century poem commemorating the death of Eric Bloodaxe known as Eiríksmál as compiled in Fagrskinna.

An etymological connection exists between the einherjar and the Harii, a Germanic people or figures from early Germanic folklore attested in the 1st century AD, and scholars have connected the einherjar to the eternal battle of Hjaðningavíg and the Wild Hunt. The einherjar have been the subject of works of art and poetry.

In the poem Vafþrúðnismál, Óðinn engages the wise jötunn Vafþrúðnir in a game of wits. Disguised as Gagnráðr, Óðinn asks Vafþrúðnir "where men fight in courts every day." Vafþrúðnir responds that (here einherjar is translated as einheriar):

All the Einheriar fight in Odin's courts
every day;
they choose the slain and ride from battle;
then they sit more at peace together.

In the poem Grímnismál, Óðinn (disguised as Grímnir) tells the young Agnarr Geirröðsson that the cook Andhrímnir boils the beast Sæhrímnir, which he refers to as "the best of pork", in the container Eldhrímnir, yet adds that "but few know by what the einheriar are nourished." Further into Grímnismál, Odin gives a list of valkyries (Skeggjöld, Skögul, Hildr, Þrúðr, Hlökk, Herfjötur, Göll, Geirahöð, Randgríð, Ráðgríð, and Reginleif), and states that they bear ale to the einherjar. Towards the end of the poem, another reference to the einherjar appears when Óðinn tells the king Geirröd (unaware that the man he has been torturing is Óðinn) that Geirröd is drunk, and that Geirröd loses much when he loses his favor and the favor of "all the Einherjar."

In the poem Helgakviða Hundingsbana I, the hero Sinfjötli flyts with Guðmundur. Sinfjötli accuses Guðmundur of having once been a female, including that he was "a witch, horrible, unnatural, among Odin's valkyries" and that all of the einherjar "had to fight, headstrong women, on your account".

In the Prose Edda book Gylfaginning, the einherjar are introduced in chapter 20. In chapter 20, Third tells Gangleri (described as king Gylfi in disguise) that Óðinn is called Valföðr (Old Norse "father of the slain") "since all those who fall in battle are his adopted sons," and that Óðinn assigns them places in Valhalla and Vingólf where they are known as einherjar. In chapter 35, High quotes the Grímnismál valkyrie list, and says that these valkyries wait in Valhalla, and there serve drink, and look after tableware and drinking vessels in Valhalla. In addition, High says that Óðinn sends valkyries to every battle, that they allot death to men, and govern victory.